I have no experience keeping sand boas, so I have been hesitant to reply. However, what Andrew said about going to an expo if you can is a great idea.
Being able to handle and interact with different species at different ages/sizes is a great way to help decide what is right for you.

Honestly, nobody can really answer that question for you. It really comes down to what you want, what your realistic capabilities are and what you're willing to put into it.

BPs are always a great choice due to their size, docile nature and all the beautiful morphs out there. However, they can be pains when it comes to food strikes. And, as Andrew said, they are often nicknamed "pet rocks" because they spend the majority of their time hidden.

I also have a corn and a hognose and honestly can't say enough good about hognose. They are incredibly personable, silly snakes. Just plain adorable!!! They are active during the day too, which is a change from my BP and my corn.

I also have to throw in my two cents on Kingsnakes. Hands down, my all time favorite snake is my Kingsnake. Super easy husbandry, eats ANYTHING ANY TIME, handles extremely well and is pretty active during the day.

So...it really is up to what you are looking for. If you've already done a bunch of research and narrowed it down to BPs and KSBs than I hope I didn't waste your time adding what I did. If not, maybe check out Kings and hognose.

Good luck, we're here if you have more questions.
Keep us posted

oh, and P.S. Sorry, I can't help with the last part of your question. I've never tried to figure out what the monthly upkeep costs me. I buy my food in bulk as well and have never taken the time to figure out what the monthly electric bill, food, etc... costs me. Maybe I should figure that out some day...