You might be able to find them for a bit cheaper, but I doubt you'll be able to find quality replacement valves for under $20 a piece. Your kites might be in good condition but they are an old superseded model, and as a result the resale value is just a bit higher than the cost of replacing all your valves...

So IMO it's probably not worth it...you'd be amazed how much better modern kites flies...any bow kite / delta kite in good condition that is even 2-3 years old would perform better than a Helix. I've tested some Helixes and they have to be some of the most poorly performing kites I've ever flown...