How much would it cost by eurostar from Paris to London?

One way is more expensive than taking a return.

Therefore I would recommend you to take a return ticket including a week-end between your two tickets, even if you don't use your return.

If you plan ahead and buy following the return/week-end rule, you should be able to get a ticket for as little as £70 return.

On average, tickets are around £100 though which is cheaper than flying. (no taxi to airport...)