I had that problem before as well..
They smallest medium rats are in 60-70g range what is actually in upper scale for small rat so.... you are basically get rip off with they 5.99 price...
They big medium rats are on average 130-140g.
Waaaaay to big for 505g python.
As Adam said.. do some research and check some small local pet stores or general feeder stores or rattery/mousery.
You'll find better variety of sizes and better prices as well.