I am in MN so costs will be a bit different. I make $12/hour at my full time job, and $8/hr at my "horse job."
My full time job pays my bills comfortably. However I wanted a horse and figured just getting one would be too tight for comfort on the budget. I got hired by a Morgan breeder to work on her farm part-time 10-12 hrs a week. That gave me 2 important things, 1. and extra aprox $400 / month. 2. With 50+ horses it gave me lots and lots of hands on with the not so joyous parts of horse ownership. I saved all the money I made at the farm and the experience was enough to let me know that yes even though there are some parts of owning a horse are not so fun it was something I still wanted to do.
Once I had safety $ saved up (aprox $2000 for any emergencies or whatevers) I made a purchase agreement with my boss on the horse I ended up falling in love with. I paid $100/month pasture board, and all other costs until she was completely paid off. While doing that I looked for a boarding barn (unfortunately Minneapolis does not allow horses in your back yard).
All said & done I found a great barn. And all though my mare is an overall easy keeper she does like her hay, and needs some grocheries on her. I pay $225/month which includes trims ($30 - I keep her barefoot), worming (~$5), & feed grain once a day & hay. She eats about 1 bale per day and that ranges from 2.50-10 but currently I think it is about $5/bale for good quality.
Oh by the way I am 5' 6", 275lbs and my mare is a 15 hand Morgan, she could go forever. You don't necessarrily need the bigger horse dure to your size. I also ride a 14.3 hand Tennessee Walker (with weight issues of her own :) ) and rides lasting a couple hours are quite fine for her as well.
One of the main things I looked for in a horse was easy keeper because of the lack of funds. I don't watch TV so not having cable doesn't bug me. I don't own any fancy new clothes,I got new barn clothes for x-mas (exactly what i wanted) I'm sure you can do it....track every penny for a month or two then you will have the answer you need. Sorry for the novel... :)