I know you workout at home so I think it depends on what you have...thinking in terms of priority. Do you have a good squat rack? If money is an issue then those things that are indespensible should come first, especially safety. A squat rack will eventually be a must.
As far as abs you can do leg raises and knee raises from a chin-up bar and you get to train your grip plus you have the added bonus of having to stabilize your body which I think adds to the effectiveness. If you don't want to hang on to the bar then a pair of those ab slings works well and
are cheap.
Ninety nine bucks isn't bad but I'd also ask how much you have dipped in the past. Some people find it pretty bad on the shoulders (especially us older guys
) so you may not be able to make a religion out of it. The point being you'd spend money on something you don't use very much. I do tricep dips off my bench (add weight with legs propped up) and I do regular dips off two high stools. Not optimal and you have to be careful but since dips are not something I do throughout an entire routine I get by with that.
That being said, dips are a fantastic upper body exercise. Tricep dips are not my favorite but regular leaning forward dips are great and of course work your triceps too. But I would try to experiment with how well the movement sits with you first.