The problem is that I'm doing 305 Mhash/s on my HD 6870 using phoenix miner but my account in mining.bitcoin.cz says it's only ~70 Mhash/s (Average hashrate in last 10 rounds: 69 Mhash/s). Sometimes when I refresh the page it goes down or up a little bit but nothing close to the 305 Mhash/s that I'm seeing on my phoenix cmd. Is this normal? Will my share and payment be calculate over what phoenix says or over what mining.bitcoin.cz says?
It's normal for the pool to be wrong about your hash rate, because it's trying to guess the hash rate based on how many proofs of work you send in, which again depends on luck. But... a deviation that large over 10 rounds, that sounds like something is very wrong.
Also I'm having nearly 0,9% of rejected work. Is this high? low? normal?
That's pretty high. Perhaps you are having issues with long polling. On my pool I had to make all sorts of adjustments to make this work well. If you have a home router that times out connections after just a few minutes you may want to change that timeout if possible.
Also, if your internet connection is unstable (comes and goes) that can cause your miner to resubmit the same proofs of work multiple times, which will cause some rejects.
Do you have any error messages from the miner?
And one last thing: Considering that I bought my hardware for gaming 6 months ago and that the money I spent on it doesn't count as a investment (I don't even knew what was a bitcoin at that time), will 305 Mhash/s at least pay my energy bill?
That depends on how much you pay for electricity and what else you have in the computer that uses electricity while it's on.