You need to factor in the cost of equipment as well. A tractor and attachments is almost mandatory if your going to be dealing with more than one horse. Something that can move bales of hay in and manure out.
For a plain Morton/Wick type building without a concrete floor approx 40x60 which is a nice good size, cost my neighbor $18,000 4 years ago. That's with one tall overhead door, one walk through door, skylights and one window. 10-15k is a good base price to start at for most Morton type buildings. Doors, windows, stalls etc. all will add to the price. Adding a concrete floor is a big expense, so you might look into only having concrete in the walkways to save money. Running water and electricity to the building is another expense.
Go talk to some of the builders in your area. Most of those guys have the prices down to an ala cart menu. ?x? Is so much with each door, window, stall etc. added to it.
The arena cost is mostly just the cost of fencing. The price of that can vary so widely by area its going to be tough to estimate. Especially depending on what you want to use. We are working on having ours all done in pipe. Its taken us 5 years to get the corners and a few line posts in
I wish I had used portable panels, they are much more versatile and I'm sure cheaper in the long run. Plus if you move you can take them with you. I think the average cost for a Priefert panel is around $50-70 for a 10' panel depending if you go economy or better quality. Just do the math for how big you want your arena to be.
Congratulations on becoming country folks