Some rooms are club level, and some are not. In general, you don't really 'add club floor access", so much as change from your current room to one that has been designated as CL.
USUALLY, the cost would therefore depends what room category + rate you currently have, and the available rate of the room you want.
CL has multiple room categories., and the room you want has to be available.
That said, at full price July rates:
tower standard view room = $259+tax= $292.
A tower standard view room with club level access = $368+tax=$414
Wow! that's only $122 for CL access, that's pretty good rate! In other pricing seasons though, the price difference will vary.
BUT, if you originally paid a discounted rate, the room you want might not be available with that same discount. Or you might have to trade Standard view for water view or a deluxe room- if that's the only kind of room that's available.
Still, it never hurts to ask what's available and what the rate would be when you check-in. I generally think CL is worth about $150/night if you have 4 people. The CL rate isn't tied to the number of guests, I just don't think 1 person will get $150 of value from Club level access. i
I is also always possible the CM can offer a little pixie dust if you booked discounted rate and want to upgrade. If you are willing to give WDW more money, you might tno have to pay full rack rates for the upgrade.