If you want cheap cartridges then you will have a job to beat the Eley Firsts. The only problem with them is that they do produce a lot of confetti at times which can put you off a little.
I use Hull Intercomps,24g and 28g,they`re very good, very clean. I use them for competitions and tuition and I love them, but they are not the cheapest, they are middle of the road really.
Use 7 1/2 or 8s or 9s for close up clays.
If you want the best then that has to be the Hull Sovereign but they cost a fortune and kick a bit.
If you want a cheap trap then look in the shooting mags, they always have some in there.