Hagen nutrafin kit works fine for me on a 40l with 28w lights. Should be able to get it for £16-17.
Sometimes the refills that come with it are out of date, and won't work much. If the ones that come with your kit work, great, use them. After that buy a small jar of bicarb and some yeast from the supermarket, and use sugar as directed, a pinch of bicarb and 1/4 teaspoon of yeast. Best yeast I've found is Sainsbury's fast acting. It comes in sachets, so when you open one put what you don't need in an airtight container. I find I have to change the solution every 3-4 weeks, and it only takes minutes.
The kit comes with an extremely efficient ladder type diffuser, but if you find this too ugly to live with you can use any other method of diffusion you like - it works fine with ceramic diffusers for example, and you can keep the 'ladder' as a spare, and for when you're bleaching your ceramic diffusers.
Lastly, although the system by it's nature is not really adjustable, one thing I do (especially in winter) is stand the fermentation container on the ballast of my lights - this means it produces more co2 during lights on than at night.
So, how much does a bag of sugar, a packet of yeast and a jar of bicarb cost? And how long will they last you? I can't be doing with working it out, for me it's fine on the 40l. It's certainly cheaper to get started with. Can you put up with changing the solution as needed? If you want a high level of control, and minimum time input then FE is still the way to go. If you're looking for a cheaper to set up system, I'd recommend the Hagen system.
(I haven't used any of the other off the shelf yeast based systems, eg Red Sea or Dennerle, they're probably all as good as each other.)
Cheers
Mark