If it's in very nice, clean shape, complete with accessories, maybe additional extras like zigzagger, buttonholer,..., then it's a good deal. Cabinet or case? If it's missing this or that it's not a good deal. If you keep your eye out and have a bit of patience I guess you will find one at half the price.
Exceptional condition (almost like new), complete with pedal / kneelever, box of accessories etc. doesn't turn up that often and expect to pay a bit more. A fix up machine (nice conditon, complete but needs a good cleaning and new motor belt is often the best buy. It's a bit more work but the price will be lower.
A clean, fully working machine like model 99 is well worth $100, but like most things vintage and second hand, you can always find a better price. It depends upon how picky you are regarding a DIY fixup, repair, any signs of age and use. It's a common model and should turn up now and then in your area. This model is usually very easy to get stitching again and will last you a life time.
A 99 is a 3/4 size straight stitcher, sews nicely and will stitch through anything you expect of a domestic machine; layers of denim is now problem. The later version had a stitch length lever with revese, the earlier had a stitch length knob and no reverse. If you make jeans, jackets and larger items it can be a bit small. On the other hand it's the size that made it popular, and it's just as capable as a full size machine. Maneuvering large bulks of fabric is easier on my 201 and it can be found at much the same price. I think I payed arond $80 for my 201, it's in a small table, and needed a few replacement parts to work again (crumbling motor belt, missing spool pin,...). I have never regretted spending a bit of time and money on it.