I have in my trade window:
5x Bull Put Combo for 5$ total. That would be 10 Puts (5 buy and 5 sell). So a total of 10 option contracts for 5$.
As well as:
5x Bull Put Combo (x2) for 14$ total. That would be a total of 20 Puts (5 B&S, and 5 B&S). So a total of 20 option contracts for 14$
I'm not doing anything special:
1) I do limit pricing (never tried market price).
2) I allow IB to route my order (SMART destination).
3) I do tend to trade in liquid options
If I am trading a higher volume (100+) I sometimes fiddle with the routing, but still have noticed anything crazy with the pricing.
For your situation:
1) Were you trading something very illiquid? (such that each contract was traded separately with a lag between it).
2) Were you trying to route it through something other than SMART (this can increase/decrease execution time and/or pricing).
I have used IB since forever, and the collection of the pricing is favourable (lower margin rates, better cash interest rates, super douper option assignment fees, low/free withdrawl fees, and I have not seen anyone with better fixed income pricing).
If taken in totality, I think IB is the most cost effective retail one out there. But if you are ONLY interested in one type of transaction, there may be better alternatives.