I have seen no-name-brand Chinese solenoids on ebay for about $20. I am actually about to order a few to try them out. (I have two reef tanks and I just set up my first planted tank)
Make sure that the ports are NPT thread. If you see "Rc", "BSP", "BSPT", "BSPP", "G", or "ISO", it means they're British Standard Pipe thread, either parallel or tapered. And you won't get a good seal without an adapter, which will be either difficult to find, or costly, or both. In fact, unless you specifically see "NPT", assume threads are BSP.