Electricity bill

I think there is no real price on like a miner. I mean I could even start mining bitcoins know.

This is just like improving your mining see this as more miners or a better like more people and better equipment.

I have a 12 GPU rig and in the UK costs me approx 240 a month

I don't think a $400 pc would do anything these days depends on what coin as well I guess.

telling you, it's too late to mine BTC now, and your $400 junk is not gonna do anything

A $400 PC will not mine bitcoin effectively. You are going to waste your money guaranteed by mining on a $400 computer. If anything you should look at altcoins. Use the computer for proof of stake coins. I would suggest doing some research.

As far as I know people get parts that are well over $400. So seeing as you have an entire build for only $400, you won't get anything. BTC is crashing and dying, mine altcoins.

Yeah i'd definitely go for something other than BTC, it is up now but it won't be this high forever. Plus your machine won't be able to mine much, not even high end machines are making enough to be worth it AFAIK

Your electricity bill will also depend on where you live and what energy provider your on. Also for some countries, electricity companies charge more for usage in peak periods so beware of that.

It depends on your rig .. and on you cost per kw ... Its most likely not profitable unless you have something big