The best, cheap filter is the DGM Optics NPB Filter. It's a narrowband filter so works well on nebulas. Remember it's not a skyglow filter though, it won't help for general light pollution and will only be of benefit for nebula viewing.
You can get these for around $80-90 shipped to your door. I have one, and they are great. Janoskiss also has one and I think a few others have recently as well. ausastronomer and h0ughy also have one though i'm not sure how much they've used it.
At the snake valley star camp, dave47tuc and I compared this filter directly with a UHC and OIII, and on some objects the OIII performed slightly better (dimmed more stars and increased the contrast), but on other objects they performed equally. And against the UHC, they were virtually identical.
Search for DGM Optics, they're a great buy.
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