We have been keeping it as office use until we get our construction financing set up along with all of the other funding sources.

It is fair to express your disappointment that they exceeded the cost target you gave them. 

If that is the case then that is considered cost of doing business and I would advise that should be calculated in your costs/returns you provide to the investor.IF you are referring to the investor who is the borrower - then I would have included that cost in the loan agreement.

It costs me $50/ PER MONTH and I can use it as a hotspot at no additional charge. 

I’ve been thinking DSCR but don’t they typically have higher down payments and closing costs?

Do you want this thing to be the tipping point for them to leave and you have turnover costs and a vacant unit? 

Are there risks or hidden costs I might not be considering?

I've added a few things for very minimal cost that guests really love and increased revenue ~34% year over year.

To estimate expenses, consider setting aside a percentage for management fees, plus costs for cleaning and utilities.

For me as well as the seller.First, you have to define Sub to financing.Do you mean the reckless kind where you overpay for a property, take over the financing and borrow from others to cover closing costs and holding costs when you have no money, no credit, no income, no reserves and can't tell a warranty deed from a deed of trust and you close on the kitchen counteror do you meanbuying below market value, already having a nice income, having reserves, using escrow and title, already understanding the due on sale clause, have done a lot of creative purchases and know when to use and when not to use creative finance and how to recover if something goes amiss?