That is a ballpoint from the Montblanc SL range, SL standing for SlimLine: a slender series of the 1980s with gold-plated trim and a coloured plastic coating, in white in this case. The SL series was positioned well below the Meisterstück and belonged to the inexpensive Montblanc writing instruments.
That is reflected in the value. According to collectors the ballpoint cost around 29.90 DM at the time, which converts to just under 15 euros. The claim heard now and again in the trade that such a pen cost 80 DM new does not hold for the SL range. For an example in as-new condition, 25 euros (as of 2010) was at best the upper limit of what was reasonable.
SL ballpoints are known with a matt coating. A high-gloss lacquered version is not attested among collectors, so caution is called for with one: it may have been relacquered, or may not be a Montblanc at all. In that case the things to check are the star on the cap top, the lettering on the clip and the action of the push mechanism.