Hi there,
sorry for another question:

I need to light a 35W SOX lamp which I need to build the circuit for.

I thought that, as ballast, I should use BSX 36 H66 and as I searched a little bit over the internet, I found very little about it and with crazy prices ranging from 120 to 850€!!!

Then I found this over product: BSX 35 5L82P and it costs only 25€!!!

Since they are both produced by Philips, both are declared as "35W/55W SOX Ballast" and both need SX72 as ignitor...what is the difference and why do they have a so different price? (100€ difference?) Indoor/Outdoor use? IP grade protection? Age?

Thanks in advance.

There's no surprise here, Philips are about to make the SOX lamp obsolete, and both lamp and control gear supplier's are cashing in on it!
The more demand goes down, the more they can charge for LPS stuff, I suspect the duel, 35/55 is more costly because of what it is?, but if you want to just run a 35,go for the cheaper one. 

The more demand goes down, the more they can charge for LPS stuff,...

Not they can, but they have to. They have fix costs running the facility, regardless how much they produce.
If the production runs in 1000's units, that fixed cost gets dissolved into the production to become negligible per unit.
But when the production dropped to units (instead of 1000's), the fixed costs share per unit becomes 1000x higher. So what was barely 10 cents now becomes 100Euro per unit. Plus 25Euro for the unit itself and immediately you have 125Euro from what once was just 25.10. This is exactly the reason, why specialty products are so expensive. And SOX is just becoming a specialty product, just there are only special applications that still needs SOX and can not use anything else (yet).
The market is apparently still willing to pay that, so they keep the facility running even in that regime. Otherwise they will just shut it down and there wont be any new SOX gear source anymore.