A bullish put spread or a put credit spread would be to sell an out of the money put and then buy a further out of the money put. You sell the expensive one and buy a cheap one (to hedge risk and reduce the amount of capital required) and net a credit and therefore the put spread is bullish. Don't bother selling deep in the money puts...there is less liquidity, you have less extrinsic value and the capital requirements are greater. Keep your spreads at or out of the money and you will do better.

Anyway to finally get to your question about the math. The bid/ask of each option is the spread of the price. The amount you will pay or receive will be somewhere in the middle called "mid-price". How much you give up trying to get filled depends a lot on liquidity and open interest. For a credit spread I usually start a few cents higher, give it a couple of seconds and do a cancel/replace and walk the price up a penny at a time until it reaches mid price...then I might let it sit a minute to see if it fills. I prefer to let the price come to me rather than just give up a couple of cents for a quick fill. When you are doing a spread you are dealing with 2 bid/ask spreads so it is important to stick to very liquid options so you can get in and out without much slippage.