I rather thought of buying a Visa and put it together with the flight expenses.
Furthermore, I think you can't fly to Petersburg from everywhere in Europe directly.
It's possible to get cheap tickets to every place in the world sometimes.
But most Europeans can reach Metz much easier and cheaper via several airports in France, Belgium, Germany.
Scandinavia, from where you can get to Petersburg easier maybe, is sparsely populated.
Russia, Ukraine, Belorus we are not talking about. (Some players from there are even in the Qualies.)
Moreover, there are several players from western countries in the main draw - in total contrast to the Qualifying draw.
So I think the main problem is money.
No, Visa is the main problem. If buying a Visa and travelling to Russia was easy, then surely more players would have played qualifying once they realized qualifying was easier than winning a Future in Rwanda? Flying to St Petersburg is not more than 250 euro from major European cities. In fact, getting to Metz requires more planning as the city does not have an international airport.
I visited St Petersburg once and getting the Visa was a true pain in the ass. I think the ATP should put pressure on tournament organizers in St P and Moscow to help players obtaining Visas on short notice.