Monte Rosa was the name of an inexpensive range of school fountain pens made by Montblanc in the 1950s and 1960s. They came in several shapes and colours, with either a gold-plated steel nib or a 14-carat gold one.
What decides the value is not so much the name as the degree of wear. Used examples in average condition changed hands for about 20 euros in the mid-2000s. Unused pens with a gold nib, on the other hand, comfortably fetched over 100 euros, and an unused transparent Monte Rosa of the 1960s brought the equivalent of just under 500 euros.
One risk of confusion is worth noting: before the Second World War there was a Swiss pen brand called Monte Rosa which had nothing to do with Montblanc and was markedly inferior in quality. Pens bearing the name Monte Rosa from the house of Montblanc were not made until after 1945. With vaguely worded listings it is therefore worth checking the nib imprint and the maker's markings before paying a collector's price.