If you want really cheap, all the Yongnuo flashes with names that end in EX can be used as a CLS slave with i-TTL, although I believe only the YN-586EX can do FP/HSS and not all the models come in Nikon versions. Yongnuo will put gold lettering on the Nikon versions of the flash, silver letter on the Canon version. And yes, even the Canon versions can be used as a CLS slave--but it'll have a Canon foot, not a Nikon one. These Yongnuo flashes come with four slave modes:
- S1: "dumb" optical, like SU-4
- S2: "dumb" optical that ignores a single preflash so can be used with TTL
- Sn: Nikon CLS slave
- Sc: Canon wireless eTTL slave
There are also CLS-slave capable flashes from Metz, Nissin, and Sigma, as well as the cheaper "eBay" brands, like Yongnuo, Phottix, Shanny, Neewer etc. Keeping up with these models is difficult and the Flash Havoc blog is one way to track what the latest models are.
There are also even cheaper all-manual flashes from a lot of the eBay brands that will include the equivalent of Yongnou's S1/S2 modes, which will work with any flash--even a P&S built-in flash--as long as there's no or only a single preflash, and you can find these in the US$50 price range. But remember, these flashes tend to be cheap for a reason. Copy consistency and component quality can vary and warranty service may require shipping the flash to China at your expense. You can go for higher reliability with a used Nikon flash that has an optical slave mode, like the SB-24, SB-26, or SB-80DX or the Strobist-recommended Lumpro LP speedlights.
And, of course, if you use manual radio triggers, then nearly any flash with physical manual power controls will work, and you won't be limited by the line-of-sight requirements of optical systems, or require an on-camera speedlight to act as master.