Perhaps I'm confused by the wording. When you say they let you buy a half point for -120, do you mean as opposed to -110 so the half point cost 10 cents? If so, this is standard and its not a good deal in too many spots, if you're saying it actually cost 20 cents then this is a horrible deal.

Buying Half Points in NFL

Half Points to for sure buy at 10 cents

3 <-- but no one offers it at 10 cents.

7 - (+6.5 and +7 buy fully to +7.5 when 10 cents per half point / -7.5 and -7 buy fully to -6.5 when 10 cents per point *don't pay 15 cents as many sites no charge*.

10 - (Buying on/off/trough 10 is about neutral at 10 cents)

14 - (Also around neutral 10 cents is very close to fair price).

Half points to buy at 20 cent

3 - paying 20 cents for this one is fine. It will be close either way depending on the total.

Buying Points in Other Sports

College Football - There are not too many profitable half point buys. The 3 at 10 cents (rare but can still be found for college football) worth it for low spread / low total. There are probably also a few spots where the 7 is worth it as well at 10 cents. Blindly though these are bad plays.

Basketball - I have no idea if this is fallacy or not (perhaps someone smarter can comment) but apparently multiple point buys stringed together increase the value of each half point in some spot? I guess this is beyond my level of thinking and I don't bet a lot of basketball so haven't bothered to really stare at that one. For me I don't buy points in basketball; at 10 cents per half point buying one or two half points is a sucker deal.