lol, I'd be tempted to e-mail back, "While I was deliberating your offer, I noticed that our undeveloped lot next door is for sale. While it's not big enough to build another house, it would substantially increase our own lot size, and in turn would be a perfect place for you and your husband to camp. Sounds like a great idea, right? Why don't you give me your bank account information so I can start the purchasing process right away?"
In all seriousness, even if location wasn't an issue, I'd really wonder about the "we're not helping buy it but we're putting X amount of work into it, "earning" our share" thing. What happens when their work surpasses the cost? Is this property earning plan constantly going? I can see this ending badly...