Doesn't appear to be much at all.
And I'm guessing you'd have a good connection.
It depends on how many slots you want, I guess.
I can really see this bring new players to Lolnet.
Surprisingly, San Andreas is still a widely played game.
I used to play SAMP all the time, and as long as you can put things into the map like stunt areas and races/shoot outs, you should attract a ton of new players.
If you're interested at all and/or plan to go ahead with this, I can give you a download link to the game (it's about 800mb if you get the one without in-game radio), which you could post on the forums for users to download in your "downloads" section.