Most enlarging lenses have fixed focal lengths, with different focal lengths for different negative sizes -- 50mm for 35mm film, 75mm or 80mm for 6x6, etc. You can find an excellent 6-element lens used for $50 or less on eBay, although prices for new lenses are substantially higher ($140-$160 for a 50mm f/2.8 Nikon el-Nikkor at B&H, for instance). Most enlargers that are available on eBay these days also come with lenses, although many of these are cheap 4-element lenses. Such lenses often sell for under $100 new, but aren't much less than 6-element lenses used.
I've never used one, but my understanding is that zoom enlarging lenses are used mainly as cropping aids or to adjust paper size without raising or lowering the head. This makes them convenience items, not necessities. I have no experience with the lens in the auction linked to earlier, so I can't comment on its quality or usability. I can say that, although I wouldn't turn one down if somebody gave one to me, I'm not willing to spend $165 for that convenience, particularly not when I don't know how the final print's image quality would compare to what I get now with my 6-element Nikon el-Nikkor f/2.8 or even my 5-element Vega-11U f/2.8.
