The rent for a croft is based on the value of bare, unimproved land. This is why croft rents are so low.
Any improvements (buildings, drains, turning rough heather into green grass etc) are done by the crofter at the crofters expense and therefore belong to the crofter (even though the land they sit on belongs to someone else). When you buy a croft tenancy a lot of what you are paying for is the improvements.
For the uninitiated it is a complicated way of doing things but there is (or was) logic. Prior to the crofters act (1883) Landlords did nothing to improve their land however if a crofter did anything to improve his croft the landlord would revalue the croft and increase the rent (even though the landlord had contributed nothing). This resulted in the Highlanders living in some of the worst squalor in Europe. If a man built a bigger house, installed running water or even put a chimney in his house the rent would go up and he ran the risk of being unable to pay and evicted (and therefore all the improvements reverting to the landowner). A massive disincentive to making things better.