The price will vary according to what you pay per Killowatt Hour (kwH). Further complicating things is that quite often the price you pay per kwH varies depending on time of use.
A couple of useful conversions to help you calculate.
1 HP = 0.75 kwH
1 kwH = 1.33 HP
The calculation is as follows: total HP used X 0.75 X number of hours per day you operate X price per kwH = price per day.
EX: Lets assume you have a 1/3rd HP compressor for your pond and you operate it 8 hours per day and your price is 0.12 per kwH at time of use:
0.33 X 0.75 X 8 X 0.12 = $0.24/day
Going a little further...when you start looking at larger systems, say in aquaculture, you will want to look the Brake Horsepower as opposed to the Nameplate Horsepower. The difference? Nameplate is the horsepower output at maximum load (in this case it would be the max PSI as a result of hydraulic head)and is named so because that is the HP output stated on the nameplate. The brake horsepower is the actual amount of energy used based on amp draw, line loss, etc... Typically the brake HP is considerably lower than the name plate. This may not be significant when your using a 1/3rd HP compressor but when you start looking at multiple HP compressors or blowers, this is important as it can equate to several hundred if not thousands of dollars a year in energy costs.
Finally, if you do operate several HP (or a lot of kwH) for any particular Ag or commercial application you may want to look into getting an energy audit from you local energy purveyor as many times you can qualify for incentives and rebates programs by switching to lower energy devices. Here in California, PG&E will pay up to 50% of the capital costs of an energy improvement project based on a set rebate price for kwH saved. Of course dealing with PG&E is about as appealing as going to the dentist.
