Oooh must be that time of year - I was just going to post this!
I'm hoping to get away with a flat level well-drained (never has puddles) piece of fenland with a membrane, some form of board/retainer and 4 to 6 inches of woodchip. NO GROUNDWORK! I understand the woodchip for a 40 x 20 will be around £1700. For me it's an issue of how well I can guess the weather as the lorries won't be able to get to the field unless it is a very hard frost.
We built ours 3 years ago (20x60). At the time we had to do it cheaply, so on top of all the varying size stone and drainage stone we put silica sand (6mm to dust) and bark on top. Bark was the cheapest option, but after 12/18 months will need topping up as it degrades. We recently resurfaced with (apparently the best sand) Cardigan sand. Wish we hadnt. Even though the drainage is good, it holds so much water and gets boggy on the tracks now. We will have to remove it and go back to bark when the weather improves. With bark, initially it can be slippery especially with an unbalanced horse/rider, but breaks down in a couple of months to a really nice surface and doesnt freeze either. Hope this helps.
Our last school was pure silica sand, it was built about 30 years ago before people really had rubber etc. When we moved to Wiltshire we wanted the same, but the contractor who was building our new school insisted on putting basalt under the silica sand, despite our protestations. It was awful and we hated it, the basalt kept coming through the sand and making the surface inconsistent. We made him come back and scrape off the sand then remove the basalt and replace the sand! It was great for a few years, but the problem with pure sand is that is quite deep when it's been dry for a while, and also prone to freezing. So three years ago we had it topped up with rubber (the type with small chopped chunks - don't know what you call it!) and it's brilliant! The surface is the same every day of the year. It's built halfway down a hill and the drainage is good, don't know how it would be if you had it in a very wet area.
Sand with rubber (short chopped stuff) on top is what we have and it's excellent. The sand on its own was horrible - very deep and very dry in the summer. We do have to level the school every week though as a track is quickly worn. We don't have any drains under the the school but it's built on porous rock so even in the wettest weather we don't get even the hint of a puddle.
A school just down the lane from us has a different sort of rubber on it. It's long thin strips that sort of mesh together - not sure what it's called - but it's supposed to be excellent (and very expensive). It has sand underneath but you never see it and she never seems to have a track worn in it either.