I don't know where in San Leon you plan on staying. San Leon in general is a bit depressed. Do a Google map street view of the area and where you are staying.
Marina Bay is 1st class but yea, a bit pricey. However, it's convenient and located in a nice safe, accommodating area. Then again, If you stay on Galveston and drive up to SC Houston you would spend the difference on gas.
I will say this: If we pulled into San Leon my DW would immediately say we are not staying here. I would then be busting tail looking for some place else :(