I'm having some issues with my 99 200hp Merc Optimax.

A couple weeks ago, I began to notice that the engine would shut down/stall out while idleing for more than a few minutes. More recently, when starting the engine, the battery/starter sounds very week and it struggles to turn the engine over, but always manages to start. Again, even more recently I am now also having the engine warning light come on when I get around 3-4000 rpms (no horns, just the light). I have had the battery checked and all looks good. I'm guessing that this is probably the starter and/or alternator. Any input guys? Any ideas on what a starter or alternator could cost? Thanks!

-Drew