Lots of cheaper options, down to and including duct taping a piece of dirty window glass to the front of the lens.
The best cheap option is no filter at all.
Here's the thing - you bought an expensive lens that delivers L-series optical performance. If you're going to put an optically poor filter on it and degrade the IQ, you could have just gotten a cheaper lens and saved yourself the money. Just sayin'.
If you want a filter, go with B+W XS-Pro Nano or MRC, or Hoya HD or Pro1 and live with the extra vignetting.