you can probably pick up a freak kit for around that price. some people love 'em (inc. me), some hate 'em. It has to be said they are not the best kit in the world, but are very flexible. the bore-sizer insert is only about three inches long, which is there biggest draw back as they could do with being nearer 6 for maximum effect. you can also get a range of fronts - not just different lengths, but different styles: spiral ported, straight ported, progressive bored, "tactical" - depending on what you like.
for the price you should really consider a Lucky barrel set. they come with a one size thread specific back, and then the barrels come in different bore sizes. the benefit here is that the bore continues down the full length, not just the start. for about £50 you could pick up a back section and a couple of barrels brand new!! they also come in a great range of colours to coordinate/contrast with your marker!
there are loads of other kits tho': evil pipe kit, dye ultralights, eclipse shaft II, redz pepperstix - to name but some.