In the UK I use the following rule of thumb:
1 adult figure = £2
1 horse = £3
1 motorbike = £4
Haven't really considered children, but £1.50 is about right.
Specials are £2.00, three packs £6 on Playmobil UK, single DS figures £2, family groups £8 which makes them less good value.
I take the value of the figures out of a set, and see if I think the reamining items are good value.
e.g. the 4271 pack with 5 Romans is currently £10, which means I get the scorpion for free.
The 4271 blue chariot with 4 horses & 1 figures is £13, so I get the chariot and 1/2 a figure for free, though the horses do not have regular bridles/saddles so I'd have to get them separately if using the horses individually.
4273 General's Tent for a whopping £20, four figures & 1 horse, leaving £9 for the tent, scorpion, and a bit of fence. Thus I've only bought one.
4277 Roman in toga for £5, only 1 figure and some props. Only ever got the one.