Reiterating the point re food sellers doing well - they always do, always have done.
I do a fair number of fairs - was at one yesterday. The card prices were between £2-4 and they were hand painted on paper or silk. So yours sound in the right price bracket.
If anyone knew the answer to the question how to guarantee to sell loads at a craft fair, we'd all be signing up to find out. The truth is - there is no 1 or 2 or 3 things that will guarantee good sales consistently - unless you make fudge or chocolates.
The economic situation at the moment isn't helping with areas of discretionary spend.
Cards can do well, but many artists offer cards almost as a side line, they will have other items alongside. There are things that help - having a variety of things on offer. If you're just offering cards and a person doesn't want a card they will walk on by. But if there's a framed picture, a mounted picture, bookmarks that can help attract people to your stand. Your display needs to be clear and attractive, and pricing needs to be clear too. Do you offer packs of cards for slightly under the individual price, eg 3 for £10. People love a bargain. Colour is important too. My main colour in spring & summer is a turquoise but it simply doesn't sell after September so I have a winter colour range - reds & darker greens.
It can be so demoralising having low sales (and I've had a few where I haven't sold a thing). Researching your fair is important - not all fairs are equal !
And then there's the whole how you present yourself - some people are natural born sales people; the rest of us have to learn how.
It always makes me smile when you see on here or other SM, people saying they are making stuff and hope to make plenty of cash selling at craft fairs. You have to run it like a business if you want to make money reasonably consistently.
Your first craft fair is a rite of passage - you survived and took £30 - that's good. Keep going, ask other crafts what works for them (and bear in mind, some are not there to many loads of money - it's a hobby, a fun way to spend a Saturday afternoon. Good luck