Transmission Leak and Check Engine Light on 1994 Chevrolet Lumina

I was wondering what the estimate would be to fix a transmission leak which seems to be caused by the gasket having been overtightened, and to see why my check engine light comes on after I am driving on the highway for a while, or over 45 mph basically.

gasket and fluid 18
labor 1 hour

the check engine light needs to be diaged for the code and failed part. a shop can do that for you.

so in a total about how much would you think that would cost, if done all at once.