Amount can vary hugely depending on stste of grass, whether the field has been fertilised or not etc.
Roughly you would be looking at between 10-20 large round bales.
As far as cost goes this too can vary - do you know anyone who might do it for you? If so they might do it cheaper thanb a contractor.
We do a bit of contracting in our area & our contracting rates are;-
Cutting - £8-00 per acre
Tedding - £4 per acre (this would need doing approx 3/4 times for hay depending on conditions)
Baling - £1.50 per bale
However we wouldn't charge less than £150.00 - £200.00 for a job like this simply cos of the 'chow on factor'! ie putting the machines on & driving to the field etc for the sake of just 2 acres isn't economically viable at normal rates. However if you have adjoining land that is being cut then you'll get it cheaper. Contractors are all different though and there may be some near you with smaller machinery than we have who would give you a better price.
The bigger ones though will either charge a higher price or not want to bother on such a small scale.
We took hay off our 9 acre field once and got about 600 bales I think but had to stack them all ourselves. To be honest the stress involved in worrying about the weather, if the contractor was going to turn up (remember everyone wants him at the same time) and then having to move and store it meant that we never did it again.
thanks for the replies, it was just an idea as i was offer the grass free for hay. but with all the hassle and the cost at £1 plus a bale it's cheaper for me to get it of the usually farmer