As mentioned above, ask the landowner, I approached the local parish council 10 years ago and they supported me to where we are today.
Allithwaite Community Pumptrack | Facebook
Get LOTS of help, they are expensive to build if done properly, and raising money is a thankless task.
A small-ish sized track with be around £150k from Velosolutions or Clark and Kent.
For comparison, ours would be around £90-100k now?
Did I mention get lots of help and support?
If it isn’t adopted by a sports body or local council, you need to factor in public liability insurance and maintenance and upkeep. Litter bins and somewhere for families to sit help as well.