Our group has eight members, and uses similar monthly / dry hour numbers to those suggested by Mad Controller, though since there are twice as many of us in the group we are raising a bit more fixed income. Obviously fixed expenditure may also vary, our aeroplane is hangared, for example, which may mean we are spending more than MC's group.
In general I agree with MC. We use about the same amount of oil but I am slightly surprised at MC's fuel consumption. We normally achieve 45 litres per hour - call it ten gallons - for going places (60% rpm, 600 mm hg manifold, 200kph). An hour's circuit bashing also seems to use about 45 or 50 litres. Presumably MC's lot like to cruise at a higher setting, nothing wrong with that if you can afford it. My habitual pattern of having 10 mins of gentle looping and rolling around towards the end of a flight has only a very modest effect on fuel consumption, but if you are into serious aerobatics, then as MC says you will guzzle a lot more.
My rule of thumb is that if you have a well set up group with enough members to keep utilisation up, and a decent Yak to start with, you will end up paying roughly about as much to fly your 50 hours a year as it would have cost you to rent a nice Archer II or similar commercially. And if you like military-style aeroplanes you do get a phenomenal amount of aeroplane and experience for your money - it would cost a lot more to fly something like a T-6, which is probably the nearest thing in terms of character and performance. Another point of reference is that it probably costs only a little more to fly a Yak than do the same number of hours in a Chipmunk syndicate, and much as I love Chippies, the Yak is in a different game altogether.