Remove the resonator! (this will increase rasp)
OR
or cut the resonator in half, and weld in place a solid pipe inside in place of the original. and drill a few holes (less then the very perforated pipe inside there already) weld it back up and you will get a deeper sound without increasing the rasp, an M3 guy I know have done this and his car sounds better than it did before, and for the £20 it cost him, not bad!
OR a new downpipe with stock catback combined with the above method! Would sound quite nice I think!
these are the cheapest ways to get a nicer sounding exhaust without shelling out alot of cash (downpipe will give more power with map to)