Expect to make about $2000 a month at OTS and $2700 a month after OTS while you are at UPT. Just Google military pay scale and you can see how much you can make depending on your rank and time at that rank.
Upfront costs for OTS are about $2000 for uniforms. You have to live on base at OTS and it is paid for. I forgot if you are allowed to live off base at UPT, but I don’t remember that being the case (I am not there yet). If you do live on base, it is free, and if you decide/can live off base, they give you a tax free housing allowance (I don’t know how much). You also get a monthly food allowance that can really save you money when you shop on base because it is tax free.
Depending on whether or not you have a lot of student loans, you should be able to afford a mortgage while at UPT. Since I have not gotten there yet, I am only going off of the expected costs and pay of UPT that I got.
And FYI, don’t be surprised if you don’t attend OTS or UPT soon. My recruiter has told me that they have accepted too many pilots in the last few years and that they are trying to spread them out by delaying some. Last I heard everyone who is accepted will wait at least a year before they go to OTS. These are only rumors but I just thought you should know in case you planned on going into the service soon right after you got accepted.
Good luck and I am sure many more will post with some more exact numbers.