OK, first things first, you need any old pot. Mine are £1.40 from Homebase. Shop around, check the reduce-to-clear section.. do you really care if it has a small chip? Be creative, use an old metal bucket, or an old watering can. Maybe you have an old serving bowl you never use or a kitchen storage container. Or even a simple wooden box. The more creative, the better.
Next you need the paint. Other than metal, which will need special metal paint.. You can use either masonry paint or wood paint. Here's the important part - buy the sample size. One tester is enough with spare to paint a pot of this size. Homebase masonry paint is £2.50 a tester, and Cuprinol wood paint is £1 a tester. I've used the wood paint as you can find a lot more colours. You may ask, why, when my plant pot clearly is not wood - Well, it may not be as robust as masonry paint, but let's face it, plant pots don't get moved often, so it shouldn't be a problem. Keep a little spare for touch-ups in later years if they need it.
I recommend three coats, with around 2 hours drying time inbetween. Once you've done the outside, don't forget to do the inside, just around the top where the compost wont fill.
Once it's dry, you're ready to fill! My tip for not spending too much on flowers, is ALWAYS buy the reduced ones, even if they look unhealthy, I guarantee you, you can nurse it back to health. Last year I bought a rose tree, reduced from £25, to £7! I gave it some miracle grow, and in no time it was blooming and is now my absolute favourite plant. Markets and supermarkets are also a lot cheaper for plants than garden centres, and often you will see "Plants for Sale" signs outside peoples houses (especially villages), these will be even cheaper. I also recommend buying a hardy plant that will live through winter. The plant I'm planing today is also a rose plant, reduced to only £1.50.
Plant it up, and you're done! You can paint a ton of these pots, all different colours and they look simply fabulous. Especially grouped together. You can be even more creative - painting patterns, or painting the trim a different colour to the rest of the pot. You can be as bright or as pastel as you wish.. You can also even paint existing indoor pots too.
Total cost excluding plant = £2.40, what more can you ask for?