There are alot of GOOD but no best champion for cheap, the best are obviously in the 3150-6300 range.
Anyways GOOD champions in the 450~1350 range are:
Twisted Fate (1350)
And thats about it for the cheap ones, as you see, alot of them are good and best in their own cathegory.
Of this list the easiest to use and efective are:
Alistar (tankish, most starting players dont like tankish)
Singed (isnt he 1350?)
On the other hand i LOVE LOVE LOVE Soraka... but I was very unsuccesful early game when i bought it so I cant recommend it to starting players. The thing is that you must be able to hold your own and a Soraka really cant do much on a 5v5 unless you know real well what you are doing and you have some team support, both of these are lacking in early players