At the moment, UK prices look like this AFAIK:
Rapier Miniatures (Ancients, Medievals, ACW, Fantasy):
24 Infantry, 10 Cavalry £1.50 GBP
6.25p/Inf Fig, 15p/Cav Fig
Baccus Miniatures (Comprehensive):
96 Infantry £7.20 GBP (£6); 45 Cavalry £8.40 GBP (£7)
7.5p/Inf Fig, 18.66p/Cav Fig
Irregular Miniatures (Very Comprehensive):
6 Infantry £0.44 GBP; 5 Cavalry £0.88 GBP
7.33p/Inf Fig, 17.6p/Cav Fig (a bit more for C20 and C21 ranges WW2 11p/Inf Fig, 22p/Cav Fig)
Heroics & Ros (Comprehensive):
50 Infantry £4.50 GBP; 21 Cavalry £4.50 GBP
9p/Inf Fig, 21.4p/Cav Fig
GHQ (WW2 & Modern):
60 Infantry £11.95 GBP; 30 Cavalry £11.95 GBP/£12.95
19.9p/Inf Fig, 39.8p/Cav Fig
Mainforce (WW2 & Modern):
4 Infantry £0.50 GBP
12.5p/Inf Fig
Scotia (C20):
50 Infantry £3.00 GBP; 20 Cavalry £3.00 GBP
6p/Inf Fig, 15p/Cav Fig
Brigade (SF):
24 Infantry £2.25 GBP
9.375p/Inf Fig
Vanguard (SF):
40 Infantry £8.00 GBP, 36 Cavalry £13.00 GBP (Microworld 40 Inf £6.50 GBP)
20p/Inf Fig, 36.1p/Cav Fig
Adler (WW2, Horse & Musket):
4 Infantry £0.32 GBP 2 Cavalry £0.35 GBP 32 Infantry or 8 Cavalry £2.75 GBP (not sure if this last figure is correct, seems pricey…it is the listed price for AWI cavalry. Most cavalry is at the lower price)
8-8.6p/Inf Fig, 17.5-34p/Cav Fig
Perfect Six (Fantasy):
16-20p per foot figure
2d6 Wargaming (Dark Ages, Feudal Japan, WW2):
48 Infantry £5.40 GBP 4 Infantry £0.50 GBP
11.25-12.5p/Inf Fig
Boki (Napoleonic French Line Inf only)
3 Infantry (0.20 Euro)
5.66p/Inf Fig
Grumbler Miniatures (Napoleonic, ACW)
96 Infantry £8.50 GBP, 30 Cavalry £6.00 GBP
8.85p/Inf Fig, 20p/Cav Fig
CP Models (SF)
20p/Fig
If price is the only consideration, it looks to me like Rapier and Scotia are your best bets, dependent upon period. However, most ranges are fairly similarly priced, so often the best advice is just to choose the figures you like best. Since P&P depends on where you are buying, I have ignored this, although it can make a difference. GHQ and Microworld (both producers of very nice figures) will be significantly cheaper in North America, I imagine.