We see these quite a bit...especially when a buyer has bought land from a seller I always recommend that they ask for a right of first refusal on the seller's other properties and they give quite often will do it for the buyer. Especially when the seller isn't ready to sell, but the seller is open to someday selling to that same buyer. Sometimes, like you stated, money is exchanged. I've seen it too many times that a seller tells someone that they'll sell it to them someday, only to sell it to someone else without telling the buyer or if they die and the property goes to the Estate the Estate often doesn't care who buys it.
A right of first refusal shouldn't have much affect on the value of the property as the seller is free to market the property and get their highest bid and then if you don't match that bid they can sell it to the highest bidder.
I hope the seller has a clear understanding what a Right of First Refusal means as I can understand it will reduce his control on picking who gets the property, but it should have no affect on the market value of the property.
It is entirely the seller's (future seller) decision to give you the Right of First Refusal, but I don't imagine it would hurt you too much to periodically check in with him regarding getting that from him or about selling it to you. It'll be a reminder that you're interested in it and considering you have already bought property from him and probably have a good relationship I'm sure you can do this without being a pest.