It depends completely on your buyer and the stats of the Blastoise. Enter its stats and level into the Veekun IV calculator (
http://veekun.com/dex/gadgets/stat_calculator ) and find out its IVs. If it has high IVs, buyers will be willing to pay a little more.
Then again, it is still entirely up to the buyers. I have had trouble selling Blastoise. I ended up selling a lvl 54 one with fairly bad IVs for 500k the other day. Players value certain pokemon, stats, and levels differently. I traded up a Pikachu for a Typhlosion, the Typhlosion for a Walrein, and the Walrein for both a Pikachu AND a lvl 97 Typhlosion. Just determine an amount you would settle for as a minimum, and then get offers.