rare pleco

its a 30 gallon planted tank mainlt corcsrcew val singapore moss and swords. im willing to spend like 50 bucks or so

You could look in to getting a L066 King Tiger. They are now on Brazils export ban list and have become incredibly hard to get a hold of. Same thing with the Queen Arabesque's.

Rare is relative to the person looking for fish. Are you talking about rare as in the most expensive fish out there or a rare fish that's just hard to find but not hard on the wallet? I don't ever see Spotted Medusa's( so ugly they look cute says my wife) or King Tigers in my area. I found some online for about 30$ plus shipping. They are smaller plecs also, staying in the 4-5" range. There are plenty of rarer pelcs out there that look awesome and cool that many folks don't have access to that are certainly not hard on the wallers.
 
Personally, I wouldn't get anything over 4" for that size tank. Here's a link to catfish that are closer to that size.
 
AquariaCentral.com