There is no fixed market price for common production models such as the Meisterstück 144. The only sound approach is to look at the prices actually achieved for comparable pens over a longer period; the 144 in particular, being a standard pen, changes hands so often that a fairly precise picture emerges. The prices do swing considerably: they tend to be lower over the summer months and around Christmas and the New Year, and the general economic climate makes itself felt as well. If you are not pressed for time, watch the market for a few weeks before selling.
The widespread rule of thumb that a used pen fetches about a quarter of its price when new is misleading, because it sets a historical purchase price against today's shop price. Montblanc has raised its list prices several times over the years; a model costing around 350 euros as of 2008 was appreciably cheaper a few years before that. Someone who bought a pen in the late 1990s will therefore often recover almost the whole of what they paid, even after years of use. Demand for the 144 has been subdued since the larger 145 came onto the market, but then it was also considerably cheaper to buy in the first place.