We replaced the Engine on our 2005 Audi A4 with an RS4 engine, and it ended up costing over $11,000.How much would it cost to change the engine in my 2006 bmw 325I?I'd look for a wrecked (hit in the rear or side) turbo 335i which would give you 300HP to start with and then do some mods that could easily increase the power to 350 or more. Dinan and other have relatively inexpensive mods such as cold air intake and chips that can bring the horsepower to the 350 mark with relative ease and cost effectiveness. I'd agree that the cost with labor will still be in the area of $7-10K. YOu could also look for an M3 engine that would put you at the 400 HP mark fasterbutt the transition would be more difficult as it's not going to be a drop in deal either.
If it were my money, which it's not, I'd think of trading the 2006 for a newer model and investing the money into the higher horsepower model already put together with some sort of warranty perhaps.
Good luck.