The cost of the bikes is minimal as I remember, but the down side is that bikes can't be kept overnight. They must be rented and returned on the same day.
I'm pretty sure we paid close t $20.00 a day for bike rentals at the WL..I would imagine the rate is the same at FW. I'd have to look at our receipt. Anyway, if you are renting a couple of them..you are nearly paying the same as the golf cart rate. Since we rented 4 of them..a few times, it really added up. I remember when they had a family plan at the marina (and I believe FW offered this at one time also) and it was $200. for the week. Unlimited rental of marina water craft, including the mouse sprites, and pontoon boats, and bike rentals. THAT was a bargain for our family of 5. I guess it would depend on how many bikes you plan to rent, as to whether it's less than the golf cart or not. I would recommend calling the bike barn directly to find out the current rates.
One thing I would recommend is if you are an AAA member, the WL marina did offer a discount, though it wasn't posted on the sign with the hourly/daily fees. You had to ask about it, and then they applied it. I would ask at the Bike Barn if they have a discounted rate for either the golf cart or bike rentals. Or at the marina if you plan on renting any water craft. Doesn't hurt to ask, and you may save a little bit.