Finding the right price

I basically charge the same for my plants than at markets. I think I should carefully increase my prices. Would you do that incrementally or at once? I, for example charged $5.50 for and alpine strawberry plant a similar item was sold (and it was actually sold!!) for more than $24 plus postage.

Some of my plants are not available on ebay so I look what other nurseries are charging. However, there is a difference between them - and it is NOT the case that the more expensive ones make no business (they are well established however, but I personally had very poor exp[eriences with them - don't tell the name here).

So difficult question - how to find a fair price?