I made this guide, because i was always using the calculator. No more!

Beer = $10 average price
Liquor = $30 average price
Firearms = $40 average price

Mike Hedges

Beer

Sells:
15 for $145 ($9.7 per barrel)
25 for $240 ($9.7 per barrel)
35 for $335 ($9.7 per barrel)

Liquor

Sells:
5 for $140 ($28 per crate)
10 for $280 ($28 per crate)
15 for $425 ($28.3 per crate)

Firearms

Mike sells the cheapest firearms in Atlantic City freshly sourced from an old US Army warehouse.

Sells:

5 for $150 ($30 per firearm)
7 for $200 ($28.6 per firearm)
9 for $260 ($28.9 per firearm)
12 for $360 ($30 per firearm)
15 for $440 ($29.3 per firearm)
40 for $1,100 ($27.5 per firearm)

Charlie Luciano

Beer

Sells:
20 for $160 ($8 per barrel)
30 for $240 ($8 per barrel)
40 for $320 ($8 per barrel)
50 for $375 ($7.5 per barrel)
60 for $450 ($7.5 per barrel)

Firearms

Buys:
4 for $200 ($50 per weapon)
8 for $400 ($50 per weapon)
10 for $500 ($50 per weapon)
12 for $600 ($50 per weapon)
14 for $700 ($50 per weapon)
30 for $1,500 ($50 per weapon)

Money

Needs clean money:
$$500 for $1,200 (240% effectivity)
$$750 for $2,000 (267% effectivity)
$$1,000 for $2,900 (290% effectivity)
$$1,250 for $3,900 (321% effectivity)

Eliza Jackson

Beer

Buys:
30 for $330 ($11 per barrel)
40 for $440 ($11 per barrel)
50 for $550 ($11 per barrel)

Liquor

Sells:
5 for $135 ($27 per crate)
10 for $270 ($27 per crate)
15 for $405 ($27 per crate)
20 for $540 ($27 per crate)

Buys:
10 for $330 ($33 per crate)
15 for $495 ($33 per crate)
20 for $660 ($33 per crate)

Ben O'Conner

Beer

Buys:
15 for $155 ($10.3 per barrel)
20 for $210 ($10.5 per barrel)
25 for $265 ($10.6 per barrel)
30 for $320 ($10.7 per barrel)
35 for $375 ($10.7 per barrel)
40 for $425 ($10.6 per barrel)

Firearms

Buys:
3 for $120 ($40 per weapon)
6 for $240 ($40 per weapon)
9 for $360 ($40 per weapon)
12 for $480 ($40 per weapon)
15 for $600 ($40 per weapon)
18 for $720 ($40 per weapon)

Samuel Johnson

Beer
On very rare occasions he takes beer (and bust kneecaps) in lieu of payment from a debtor...

Sells:
13 for $100 ($7.69 per barrel)
17 for $130 ($7.65 per barrel)
24 for $180 ($7.5 per barrel)
Firearms Edit
Samuel doesn't like to use the same gun twice when 'hurting' his debtors, and he must have a lot of debtors because he's often got used firearms to sell to you...

Sells:
2 for $75 ($37.5 per firearm)
4 for $150 ($37.5 per firearm)
7 for $260 ($37.14 per firearm)
23 for $720 ($31.3 per firearm)

Money

Offers loans:
$250; repay $400 (10 payments of $40)
$500; repay $750 (10 payments of $75)
$1,000; repay $1,500 (10 payments of $150)
$2,000; repay $3,000 (20 payments of $150)
$3,000; repay $4,500 (20 payments of $225)
$5,000; repay $7,000 (20 payments of $350)

John J. Gulf

Liquor

When buying liquor, John makes half the payment up-front and pays the remainder on task completion. Just be wary of the unit price he offers for 80 crates!

Buys:

5 for $180 ($36 per crate; 2 payments of $90)
10 for $360 ($36 per crate; 2 payments of $180)
15 for $540 ($36 per crate; 2 payments of $270)
20 for $720 ($36 per crate; 2 payments of $360)
25 for $900 ($36 per crate; 2 payments of $450)
30 for $1,140 ($38 per crate; 2 payments of $570)
80 for $3,000 ($37.5 per crate; 2 payments of $1,500)

Money

Needs a loan:
$250 loan for $400 (10 payments of $40)
$500 loan for $750 (10 payments of $75)
$1,000 loan for $1,500 (10 payments of $150)
$1,500 loan for $2,250 (10 payments of $225)

Lady Cunningham

Liquor
Warning: She often wants you to buy her a present with the money she gives you.

Buys:

5 for $225 ($45 per crate)
10 for $450 ($45 per crate)
15 for $675 ($45 per crate)
20 for £900 ($45 per crate)
25 for $1,125 ($45 per crate)
30 for $1,350 ($45 per crate)
35 for $1,575 ($45 per crate)

Firearms

Buys:
10 for $1,600 ($160 per weapon)

Deputy Kingsley

Beer

Sells:
15 for $150 ($10 per barrel)
25 for $250 ($10 per barrel)
35 for $350 ($10 per barrel)
45 for $450 ($10 per barrel)

Liquor

Sells:
10 for $300 ($30 per crate)
15 for $450 ($30 per crate)
20 for $600 ($30 per crate)

Special
Offers to reduce heat by:
1 star for $150 ($150 per star)
2 stars for $300 ($150 per star)
3 stars for $450 ($150 per star)

Rabbi Hobsbaum

Beer
Warning: He will often try to haggle the price down upon task completion.

Buys:
15 for $170 ($12 per barrel)
25 for $300 ($12 per barrel)
30 for $360 ($12 per barrel)

Liquor

Buys:
15 for $495 ($33 per crate)
20 for $660 ($33 per crate)

Money

Offers to launder money for you:
$$180 for $300 (60% effectivity)
$$300 for $500 (60% effectivity)
$$800 for $1,000 (80% effectivity)
$$1,700 for $2000 (85% effectivity)
$$2,300 for $2,500 (92% effectivity)
Note: an Accountant ratio is 3:4 (after upgrade); thus only last 3 offers pays out

Mr. Smith

Liquor

Buys:
5 for $135 ($27 per crate)
10 for $270 ($27 per crate)
15 for $405 ($27 per crate)
20 for $540 ($27 per crate)

Money

Mr. Smith pays the biggest commission fees in the game for your money laundering services, enabling you to make huge profits if you have sufficient clean money to meet his requirements. Charlie Luciano offers better ratio (for any but $$500 to $1200 offer), actually making his offers more profitable.

Needs you to launder money:
$200 in to $$80 (250% effectivity)
$500 in to $$200 (250% effectivity)
$1,000 in to $$400 (250% effectivity)
$1,500 in to $$600 (250% effectivity)
$5,000 in to $$1,500 (333% effectivity)

Joseph Springfield

Beer

Buys:
20 for $400 ($20 per barrel)
30 for $600 ($20 per barrel)
40 for $800 ($20 per barrel)
50 for $1,000 ($20 per barrel)
150 for $3,200 ($21.3 per barrel)

Money

Offers to launder money for you:
$$150 for $250 (60% effectivity)
$$275 for $500 (55% effectivity)
$$425 for $750 (57% effectivity)
$$650 for $1,000 (65% effectivity)
Note: an Accountant ratio is 3:4 (after upgrade); thus none of the offers pays out

Alexy Lieven

Beer

Sells:
20 for $210 ($10.5 per barrel)
30 for $320 ($10.6 per barrel)
40 for $430 ($10.8 per barrel)

Liquor
Alexy offers a unique liquor deal - if you can loan him some liquor now, he'll more than double it in return!

Deal:
15 for 40 (+25 for free; give 15 now, he gives 40 on completion)
Sells:
5 for $125 ($25 per crate)
20 for $500 ($25 per crate)
25 for $625 ($25 per crate)

Firearms
Sells:
5 for $175 ($35 per weapon)
7 for $245 ($35 per weapon)
10 for $350 ($35 per weapon)