i dont understand why you have asked how much it costs to run a month, as my running costs will vary widly to someone doing 16k a year? are you asking how much petrol is/what mpg we achieve? if so mine achieved 230 miles to £25 other week, mine is standard and that was a motorway run, if you are talking town driving, booting it i can get around 26/27/28 combined but i dont drive that harsly. i think that mine is excellent on fuel.
additionally i feel valvers and willliams have a bad rep for being unrelieable and expensive to maintain. i have had my williams a year and i dont feel that if is at all expensive to run, and it has never broken down, it has had a couple of issues like the abs light pissing about but thats it! nothing else in just over a year. although, it must be said, alot of owners have had more than their fair share of issues, i think i have just been lucky...
however the car is very raw, there alot of rattles and noises, in fact everytime i get in the car i spot/here a new squeek, and imo the cars need more than a little tlc, for example mine gets a VERY gentle run from cold, and the engine can sound a little rough in the morning. if you are coming from a "newer" car you will have a shock, although looking at your avatar i presumed you had a valver anyway, or is it a lookalike.
anyhows good look if u need some more help let me know