At minimum, expect to spend $2,000 for the part and about $1,000 for a mechanic’s labor for a 2010 Impala transmission replacement cost.
It sounds like your mechanic quoted you an appropriate cost for this repair. Sorry you’re dealing with such a costly issue! Unfortunately, transmissions are one of the most expensive parts of your entire vehicle.
To save a bit of money, ask your mechanic about using a rebuilt or remanufactured transmission. This is usually a bit cheaper (and just as safe) as a new part. Of course, get quotes from several other mechanics in your area before you book this repair.