That is a high cost for an extra 5GB per month. It sounds like you might be on the same plan I am, and the only higher plan available is the one for $129.99. Quite a leap in price for not much extra allowance, though the speed is a higher up to rate.
You may already know this, but there are a lot of things you can adjust so that you don't go through a lot of data. Of course, if your reason for wanting to upgrade is to be able to watch on demand films and TV shows and such, all the adjustments in the world aren't going to help with that, save for watching said films/shows in SD.
My 15/50GB plan is actually overkill for me. I chose that only because I figured I would need it, plus I got $10 off per month for a year. As it turns out, I would be fine with the 10/50GB plan, and could actually squeak by with the 5/50GB plan, if I needed to. I just make sure nothing is automated that doesn't have to be, and I don't use the cloud or syncing. Also, this is with three PCs running Windows 10.
I hope you can adjust your computer(s) to be able to fit your existing plan.