Hi!
We chose the highest part of our property then scraped it flat with a 1950s tractor. Never laser leveled, or did anything fancy like that, just watched the water pool, drain, and run. Then added 5/8" minus (screenings) to any areas that were mushy or puddled. Used the tractor to flatten this and drove over it a bunch. Rinse and repeat this process for another month or so during winter – ending up with about 1/3 of the arena with patches of base. Added a full 10 yard load of gravel to the lowest end of the arena as it was the mushiest and comprised of the most fill vs. previously compacted dirt. Used a vibrating riding rolling compactor on everything including the dirt. Then added coarse washed sand, approx 2" of it and compacted this as well. Horses do occasionally kick up the gravel or hit the clay base, but they are smart and learn from it 
Glacial till is the sandy, rocky, clay mixture soil that we have in the PNW. It has pockets that drain perfectly and others that don’t.
I LOVE my arena for the price. All in, including three board fencing and stain it was $4500 ish? We usually have 6-10 rides on it per day and I feel fine jumping up to 3’6" in it (but that has more to do with the size of the arena than anything else). I also only have a chain harrow and a quad to drag it and that works for the most part. Again – this is not the perfect set up by any means, but it is MORE than sufficient for a home arena and is safe and suitable for my horses and needs. The cost/benefit on putting in a dream arena wasn’t there.
