Re: evinrude lower unit
The short answer is ALOT.
A rebuilt lower can run between $600-$1400, try looking on Ebay, they usually have some and that is a pretty common engine so you should be able to find one for way less than a rebuilt.
Generally lowers fail from either prop strikes or water intrusion, water makes a very bad lubricant.
The water in the piston is a head gasket, if you have decent mechanical skills you should be able to handle that by yourself, not to hard.
As far as cost I do all my own work so I do't know what a mechanic would charge.