This is not the right conversion rate. Like with all conversion abilities you pay extra for the opportunity to choose what kind of resources you really want right now. If the ability would go the other way round, fuel into manpower, you wouldn't definitely get your 200mp back from 50 fuel. Remember how expensive the conversion on okw mechanised was? This is similar.
Btw, the cheapest LV is probably kubel.
Well, you build a cache for 250mp and get a steady flow of fuel. It adds something like +5 right? So 250mp +10 min 50 fuel. I think the conversion stands.
The conversion on mechanized was muni to fuel and was applied to a reduced income so it was kinda fucky (although I do miss it) I would say that because it was a strange and constant ability on a unique income design with a faction designed around it that it ranks as an outlier.
Also I specifically didn't mention the Kuble because we're trying to use a fuel cost to determine an approximate value in manpower when fuel is considered. But for the sake of considering all bases the Kuble costs 210mp when we convert the fuel 210+((4)0)= 210 so we can determine that with the fuel conversion applied the Kuble true cost is 210mp. Hope this helps.