Have you guys considered using rainbird sprinkler valves? Typically they sell for about $20 a piece but they will require reducers to 1/4" lines and a 24VAC adapter. So they will be a little higher than the budget but I'm not sure if you can find a wholesale lot of them and save some money on a group order.
The reason I suggest them is they may be more reliable.
- They have a 5 year warranty
- Guaranteed to fail in off position
- Been around a long time and widely used
- The solenoid can be replaced very easily
- All parts can be found locally if required
The valves also allow you to manually open them when required and have a bleed valve. I've used these valves in many top off and water change systems, including my own, and have never had an issue. Some are well over 5 years old.