Unlike Ivar, I assume you mean Lyon, not Leon Whichever, your best bet for the cheapest is probably via either the UK or Germany, which countries have the most budget flights. One site which might be useful is http://www.whichbudget.com/en/ - via UK try http://www.flightmapping.com/maps/Europe.swf
There are quite a few of these overview sites around but it's very difficult to keep up to date with all the routes/airlines, not so much the new routes which are generally announced with fanfares, but the routes which are quietly discontinued.
As usual, best for cheap fares is off-peak (not weekends and public holidays) and book well in advance.
Feeling like a travel agent, and knowing France quite well, I delved a bit further into this. Peter is quite right, flying from Norway via London Stansted will be the cheapest.
Ryan Air flighs from Oslo Torp to Stansted for about 80 NOK, from Stansted to St. Etienne for about 15 pounds, which translates to a total of about 35 Euros. From St.Etienne which is closer to Le Puy than Lyon it will be about 12 Euro by train to Le Puy. Bus transportation in rural France is usually not very extensive or convenient. Ryan Air does not fly to Lyon or Clermont Ferrant, the other 2 major airports close to Le Puy. There might be others.
But you have to consider connecting times in Stansted and land transportation in St.Etienne.
I bet this Stansted route is the way to go, if you book early enough!