I 've purchased plants from such events, and found that an easy to grow
perennials, such as geraniums, which can invade a garden, and hostas,
usually sell for 50p to 70p, I noticed that small rockery plants were 3 for
£1.00. Anything a little unusual is a good sell, as most people are looking
for something different. I noticed that shrubs sold for £1.00 to £1.50
depending on size.

Also noticed pots containing a few herbs, eg different coloured sages,
chives, variegated mint etc, seemed to be a hit.

At this time of year, bulbs would be a good idea. Trailing Ivy etc. and
heathers.

If you have a Morrisons near you, have a look at their prices.

Hope this gives you some idea