I built my wife an arena last year.
It totalled £16k plus 10 days of my time that's cheap for a big arena
I did a fair bit of excavation, built massive drains and didn't scrimp on materials but it's worth it when it rains and she has no puddles
I recall moving about 500 tonnes of soil but a flatter site would have been much less(we live in national park so needed to negotiate position with the planners!)
Limestone was £400 per 20 tonnes for correct size
(Slightly cheaper if you can get an artic near)
I used £6000 worth to ensure good drainage for similar arena
(You can use crushed brick and it will probably drain well enough)
Silica sand was around £4k, rubber £1.2k
Membranes and drains £1000
Post and rail/running boards and contractor to sink posts £1200
Rest was gate, lights , landscaping, hedge(condition) and diesel(have my own digger tractor and trailer)
It ain't cheap if you do it properly
Company will charge you £25k+
6k? If you have a flat surface, get rock from a demolition(brick),
Get a deal on membrane and only use between rock and sand.
Get cheap sand and rubber doing all the work yourself without hiring machinery and the trucks can get to the site(do it in summer) you might have a chance.
Let me know if I can help