If you've only £50 then your going to be looking at something pretty basic and you might be best going to your local pet store to see what deals they have. If your looking at a coldwater setup, then you might be able to get a slightly larger tank as you don't need things like heating and lighting, where as a tropical setup you’re going to need a heater and possibly lighting. So this might force you in to a smaller type of aquarium often refered to as a Nano Aquarium.
Remember that your going to need items like TapSafe, Food, Gravel and possibly a filter if one isnt included, so remember to budget for this as well. Larger pet stores will often do starter packs for fish tanks so always look for these first as they are often cheaper.
If you've got £1000 then you could be looking at getting a nice Marine Aquarium setup, but in turn this will need more looking after and more equipment, such as protein skimmers and a wider range of test kits, although most items can be bought at a later stage.
If your looking at keeping your aquarium on your kitchen work top in an unused corner then you might be best at looking at a marine nano aquarium where all the equipment is often located in the rear of the compartment in hidden compartments. If you have a large wall space where you are looking at placing your aquarium then make the footprint of your tank that you might be purchasing, as fish tanks often look smaller in a large pet store and then you get home to find half your lounge gone. Remember to check you can get your tank into your house.
3. Your Knowledge
If your new to fish keeping then you might find keeping marine fish hard and mistakes can often be expensive with fish holding a higher retail value. Clearly you might want something more flash than a traditional Goldfish in your new aquarium, so if your setup is right then take a look at Tropical fish keeping. If your coming back into the fish keeping hobby then maybe dabble into marine fish keeping.