The cover is one of the most important features a book can have. It catches the eye of the reader and will often be a major influence in their decision to read it or not.
Self-published novelists have it easy. This is often a no-brainer decision: buy the best cover you can afford. When writing short stories, this becomes less clear cut. You have to look for both quality and quantity. If you're putting out a new short story every month, it is simply not practical to be forking over $100-300 on every one of your covers. Especially considering that short stories are more niche. So it will probably take a long time before any one of them start to sell really well. At least enough to break even on the cover.
But, at the same time, a good cover can increase sales. What to do? In my opinion, it's critical that a short story author becomes familiar with Photoshop (or some similar software). Unless you have the money to drop on good covers, spend a headache or two on learning this skill. Think about it: spending $100 on a cover would mean you are out $1,200 by the end of the year if you release a new short story every month. Spend $150 at a jr. college and take two semesters of a Photoshop class. It's a worthwhile investment.