If you take the approximate weight of the sized horse you plan to buy and multiply that weight by 1.5% and 3% you will get a range of the daily hay needs of that horse. 1.5% for a really easy keeper and 3% for a harder keeper. Maybe a touch more if you are in a really cold environment during the winter. For example - a 1000 lb horse (just using this for ease) would be 15-30 lbs of hay a day. If you have 60 lb square bales you would be feeding 1/4-1/2 of a bale a day so about 7.5-15 bales per month. That is as close as you will get until you get the specific horse in question. This doesn't include any extra supplements/pellets that a hard keeper may need as well.
Also - if you are worried about expenses, why are you looking for such a large horse? With full drafts and a lot of crosses the tack expenses are more as well just to get something that fits properly. If its what you are really interested in and just love them - sure save up for it. But if its just because that is what you think you need - you might be surprised at what is out there that could easily meet your needs and carry you that might not be such big hay burners and meet all your needs.
For example - I'm 5'9 and over 200 lbs and I ride a 13.3 hand haflinger which includes some strenuous long rides up in the hills that can last 6 hours. I've had him his whole life and he does just fine. However, I don't think I would jump him with my size, but if I had a larger haflinger - 14.2-15 hands jumping would be feasible considering their weigh carrying build.
My husband is 6'4 and has been up where you are at at one point and he rides a 15.2 hand draft cross. Its still a draft cross and plenty wide to take up leg, but smaller than some of the huge draft crosses out there so doesn't burn up as much hay. Just something to consider during your horse search.