I'd assume the biggest cost is getting the soil fertility right to make the new hay grow?
Does your soil need lime, ph is the big thing, neglected hay fields go bad in a hurry and need a couple years to get them back ito a good ph, you'd want to start this fall already on that.
P & K would need to be addressed, and best to put 3 years worth of those worked into the ground this spring is the most efficient. That is the big, big $$$. Again, a big long term cost for hay, and an old abandoned field will likely be quite low in at least one of these. $$$$
Then, by 'hay' are you talking all grass, all alfalfa, or a mix?
Here in MN, it is a poor plan to try to plant a grass or alfalfa crop in the warmer months of summer, they need to be planted in the wetter cooler months of April or early early May, or again in late September. Donno what timeframe you were looking at with the 'one cutting' of oats but if you grow oats, no need to hurry up and plant hay, wait until your cool and rainy fall timeframe.
It takes a year to make a good growing hay crop, so if you are putting in all the expenses, be sure that 'free' rent is covered for several years, not just one.... Would be difficult to pour the time and money into totally renovating a hay field and recouping the costs in one year.
If all you want is a grass crop and ph isn't an issue in your area, often one can just manage what is growing with good fertility and properly timed mowing to rebuld the grass already there and proper mowing will bash down a lot of weeds - another option to look into. Can work better in a dry year, which has been an issue for many of us, fall has been too dry to establish an alfalfa crop here 2 years in a row. Fertilize well, light tillage to scratch the fert in but only killing 1/4 the sod or so in early spring - maybe throw a little good seed on and drag lightly to get the seed to catch in the 1/4 killed areas, and hope for rain, mow properly to let the grass grow but not the weeds going to seed, and you got a good field by fall with a couple cuttings off. This would be the easiest cheapest if you want a mostly grass end result, and your poil ph isn't much low.
I didn't help you much with actual costs, which is what you asked, sorry, hope it's some things to think on and refine the question?
--->Paul