I picked up a Gameboy Player (w/ disc) today for $12 so I will be getting rid of my red SP if anybody is interested before I put it on ebay. Just send me a PM with what you are willing to offer for it (trade or cash). I am mostly interested in N64 stuff but I am open to any game related items.
If the red SP is your only Game Boy Advance model, I would suggest keeping it. You don't want a Game Boy Player to be the lone method of playing Game Boy, GBC, and GBA games.
Unless you have a Game Boy Color and a Nintendo DS Lite, in which case... well I would still keep it, dude. Even if I had a GBP, I would still use my cobalt SP for Game Boy playback at the very least.