Xplosys is right.
Model pack 4 = £11.99
Lets say it easy makes 500 sales.
Total : £5995
21 models in the pack. so each model sells for 5995/21
= £285
Even if TGC takes 50% of the sale price, the artists still stands to make £142 on each model, if the pack sells 500 copies.
It's an economy of scale. The price is cheap to the individual, but because of a bulk of individual purchases, the overall profit returned is high.
If someone is going to be making individual models for you, they are only making one sale, and therefore need to re-coup the same price in 1 sale. Meaning you pay alot more.
Sure you could give the artist the right to resell the models he's making for you elsewhere, but he may not want to, or have any need to, so really you're at their whim.
£100 will honestly not get you very far. That's £50 a character, which is less than 5 hours of work on each character. Barely enough time to model it, let alone UV map it, texture it and rig it.