I think the Retro Ronde probably manages to keep its price down because there is almost certainly financial and other support available from the local and/or regional government for this type of event. They have realised that the Tour of Flanders is the biggest tourist draw to the area and are willing to invest in making the most of it.
The Belgians are generally quite good at supporting enlightened government investment to kick start tourism. The recent La Savoureuse seemed to follow a similar model, and in Limburg there is a subsidised scheme for tandem hire which attracts people to the region. They then spend money in the restaurants, hotels...
In the UK, since such an enlightened approach doesn't fit with our political culture (slash and burn!), there seem to be two alternatives. Either start out as a 'pirate' event, doing things in a cheap, informal way, and hope that local government people are wise enough to recognise the potential benefits (i.e. what l'Eroica did), or organise a shamelessly commercial event and try to turn a profit.
That is exactly my thought.
Have you been to the Savoureuse?
I went to La Savoureuse and I thought it was fantastic. Having missed out on the early days of L'Eroica and RRVV, it was great to be able to take part in the first edition of an event. It was very small, only 150 people according to the organisers, but the atmosphere was great.
They had some really nice locations for the food stops, particularly the first one in a castle with champagne and foie gras
There was also plenty of Belgian beer flowing. One guy even offered to fill my bidon with Malteni beer!
Like the RRVV, they put a bit of effort into making the food stops interesting, e.g. with live musicians or stops in weird museums.
The parcours was also really good, barely any flat sections, lots of climbing and descending. No real monster climbs, but enough to feel you'd had a decent days ride. Also, the location in Luxembourg province is quite underpopulated, so you really feel you're out in the country.
If you want to do this ride, I'd suggest coming over and doing some of the climbs from Liege-Bastogne-Liege and the Fleche Wallonne on Saturday and then doing La Savoureuse on Sunday.