Take tomatoes and onions. You but two packs of seeds which will cost you no more than a couple of pounds. You sow from these 20 tomato plants and 150 onions (even then you will have only used about a third of the seeds !).
When they're ready, rather than selling them at the gate, you make chutney and relishes from them and sell on the gate, at boot fairs or simply friends and their work colleagues.
Look at it as; the seeds cost me £1, if I sell them 'out the ground as they are' they'll bring me £20, but if I make them into chutney I'll get £100 !

Then there's natures freebies such as horseradish - costs nothing, but made into sauce which is an easy and fast job, it's 100% profit. There's wild garlic, wild strawberries, blackberries, elderberry, all sorts of things.
Even stinging nettles can be eaten or made into hair tonics, as can rose hips.

A good look and read of a wild herbal recipe / medicinal / useage book is quite mind boggling.

Or just taper off one area from your garden and grow the same thing in it and sell that, flowers even.

The thing is that from a working point of view, the garden is mainly spring, summer and autumn work, but the actual results of your hard work are to be had all the year round.

From Home
The list could be endless, but if you have a skill and like doing it - use it to your profit.

Here's a list of some things either I've done or other people who I know have done....