I've tried to look this up via google but with no luck. What was the cost, both in coinage if it was used or in trade, to mill grain? What would the miller have charged? I know that in medieval literature, Chaucer for example, millers are often portrayed as being corrupt or greedy. But what does that look like in terms of what you would pay the miller?
It's hard really to give a definitive answer to this, as it's a bit like asking "How much does an MOT cost?" That is to say, while there might be a rough average, the 'cost' could be very different based on the ownership of the mill, the customer, the location, the period and the scruples of the miller.
Mills could require significant work or capital to establish and maintain, so were typically the property of land owners rather than individuals. Millers, however, might hold a mill as a tenant, especially if the landowner wasn't present in the community, or may have been, essentially, a paid employee with a stipend from the landowner. The actual fee to use the mill was then the prerogative of the miller or the landowner. This typically took the form of a "multure", a payment in kind of a portion of the flour ground or, potentially, payment in lieu thereof. In essence, this is an effective way of ensuring that the miller and his household have a guaranteed income of food and that people could afford to use the mill. Actually establishing what the size of that multure was is a harder affair: multures changed from estate to estate, or potentially from tenant to tenant; freeman peasants are likely to have paid a lower multure than tenants, for example, and multure rates could well have been part of tenancy agreements for specific holdings.
Multures are discussed in contemporary documents such as manor rolls or court petitions but these typically deal with multure rights rather than specific values, although there are some exceptions: a 1381 petition to Parliament from the mayor and commons of Chester (SC 8/102/5083; Parliamentary Petition 2048) for example establishes that by royal charter, the mills in the city of Chester were entitled to "the right to multure of a fifteenth part of all the grain milled" therein. If you wanted to put this in terms of a hard and fast estimate of a monetary fee, using an incredibly approximate estimation of land value and size from Domesday Book, a 'villager' or villein household in possession of a typical virgate of land might pay some 4d annually to the miller for use of the mill.
Accusations of theft by millers are found in the legal record across the Medieval period. Typically these deal with dishonest weights and measures; e.g. the miller might use over-weighted weights to claim multure of two kilos of your flour while, in fact, taking three; or under-weighted weights to give you 'five' kilos of flour which is in fact only four, and keeping the remainder for himself.