By linking London to Paris in 2 hours 20 and Bruxelles in less than 2 hours, Eurostar has transformed the inter-channel travel since it appeared in 1994. It is now the leader of transport market between the 3 cities and carries more passengers than planes and ferries together.
Several fares are available. By booking in advance (from 3 months), it is possible to find fares from £59 return ticket from London (and 70 EUR from Paris, as rates depend on station of departure!). However those rates (except specific deals) are rare and it's more common to pay £100 or 120 EUR.
Even if some airline deals seem cheaper for the same destination, you must consider the price of getting to the airport or to the city centre as airports are often far from the final destination.
You will also notice that it is possible to split tickets, i.e. to buy a first class ticket one way and leisure class return for example.