You mention field grasing at $15, but unless there is enough grass year-round you will need to pay for hay and supplements. Also, unless the field has a suitable horse shelter, this is only a solution for good weather (e.g. not wet days in winter).

How about costs for a stall (including straw and mucking out)?

Vet costs are a big unknown. Might only run you $200 a year, but if you get a bad case of colic you could spend thousands overnight.

Also, costs change. Some areas are reporting recent hay prices double or even triple what they were a few years ago.

You need to go through the list of all horse costs and see which are applicable to you. Then phone around to check local prices and calculate the total (the link in the previous sentence has 2 on-line calculators for monthly and annual costs). Then add 25% as a contingency.