Hey Christian, Don't fret about your cash flow situation, you can pull it off!!!
There are a couple of things you need to know though:
London and Paris are 2 of the most expencive cities in Europe, this cost of living and traveling can really set you back. With 500$ you could pull off prob one of those cities but because of the cost of transport between them, you might find it hard to do both. BUT if you can get a sweet deal from:
You could end up getting to where you wanna go with very little cost. If your not picky about when you fly you could pay nothing, and just pay the tax.
If you would like a suggestion, fly into Amsterdam and travel by train to Paris, then you might be able to afford to hit Luxembourg and some border towns in Germany, then make you way back through Belgium and take the ferry to Dover and into London, flying out from there. It will not be an easy task with the amount of funds you have...but being frugle will make it happen.
Try and hook up with some locals and if you lucky that would cover some nights of lodging and some meals...and if you feel safe to hitch hike then that would save you costs on transport. The hitch hiking thing is risky and not what it used to be back in the 70's, but some peps still do it.
One thing to remember is to try have a credit card available to you if you run into an emergency.