What's a fair price?

I am going to try selling sweet peas this year, but I'm unsure how to price them. Generally, I undercut the big online gardening companies and garden centres because I think they're overpriced. I've done a bit of research and the online companies sell them in modules of 6, 10 or 12 cells. A lot of them are multi sown, so you get 3 or 4 plants in each cell. 

So I'm thinking of sowing 3 to a cell in 10 cell modules, but I don't know how much to charge. How much would you pay for 30 sweet pea plants? The range online seems to be between 50p and £2 per cell - £20 for a 10 cell module seems way too much, even though it would get you a lot of plants, and £5 seems more reasonable.