Kaz, have had lobster a couple of times in Goa, and although nice and always well presented, for better value and taste is crab. Also try the jumbo tiger prawns - and I mean JUMBO they are massive and not seen them anywhere else, 3 is a decent amount. Prices vary according to the fish market rates/size, Lobster expect to 500rps-800rps - crab/jumbo prawns 400-600. Beach shacks, if they have them in then fine, but quite a few do orders for the following day (at least something else to look forward to !) Restaurants will always have them, unless sold out of course.
Curry's, well you can't go wrong, but tell your other half not to expect all places doing them the way you get them in the UK. These are laden with full, fresh spices and if you go down with the dreaded delhi belly, it is quite possible the spicy food (and too much) rather than 'bad food'.
An average price for 2 people would be around 700rps for mains and starters and a couple of beers. You can pay less than this, but you can also pay a heck of a lot more.
Read the reviews for recommended places on this site, someone else's choice may be a let down for you. Too many places to mention for me, have fun finding them !
Spike.