I have one implemented in my (unreleased) API.
You can use it here though:
http://chaosagent.org/10-5api/api/?command=getProfit&timeframe=day&hashrate=1000&watts=10&kwhcost=0.1

the timeframe option in the URL is either hour, day, week, month, or year
hashrate is your hasrate in kh/s
watts is your power consumption
kwhcost is your electricity cost.