How much it will cost me?
Depends of each work, but i can say, a simple interface without too much logic on it, could be done for 10 - 30 bucks.
How long you will spend to deliver me assignment done?
Depends on the amount of the complexity on your app, but i usually deliver some simple works in 12 - 48 hours.
And if I find a bug (error) or something wrong on your delivery?
You can be sure, i will resolve any errors you find on the delivered code, if it related to anything i did, or changed based in the original assignment we had negotiated.