There are a few variables that can determine the cost of replacing a transmission in your Blazer. If you can find a used transmission at a junk yard, or buy a "totaled" Blazer for scrap parts you can get a transmission for relatively cheap, say around $200-$300, if you are capable of doing all the mechanic work yourself. Or, it can cost you upwards of $1500-$2000 for a new one installed by a certified transmission specialist.