Hello Fernando,
Yes, that's true because the 5,196 250ies were only built within a timespan of roughly 13 months and not for 3-4 years like the others, respectively.
Basically the M129 (250's engine) were known for high oil and gas consumption in comparison to 230 & 280 in the sixties. That's why their value is still under"value"d even today.
I don't know whether that is really still justified today, most of today's 250 SL (and SE) owners are happy with their car & engine - basically not more or less than 230 & 280 owners.
Please read also the buyer's advice on this homepage and several of the threads (search mode).
Good luck !
Achim
(Magdeburg, Germany)