The cost depends on many factors.
If you're an older candidate, it'll take you longer to achieve and therefore cost you more.
If your 150 hours were constructive and challenging, you'll be better prepared than you would be if all your trips were short hops in the local area
Don't underestimate the cost of approach fees (often mentioned only vaguely by the FTO). You wouldn't believe what some airfields charge for a few shots at the ILS.
It's fair to say that an IR is very rarely achieved within the minimum cost/hours/time advertised by an FTO.