Er, nope, sorry! Fish are expensive and fairly high maintenance. They are quite sciencey too and they don't do much so they are not ideal pets for children.
The cheapest set up I could think of would be a small-ish tank approx 12 gallon (yes, this is small in real terms, those little toy plastic tanks are rubbish and cruel for keeping fish in), with a small box filter and a small shoal of minnows which don't need a heater.
All that would set you back around £60 just for the tank and filter if you bought brand new but there's a lot of second hand stuff knocking around like you say at car boots. Freecycle usually has plenty of fish tanks up for grabs too.
Along with a tank and filter, you'd need a water testing kit, dechlorinater, basic meds, food, lighting and decor. It all adds up! Plus there's the running cost of the filter which cannot be switched off unless you are cleaning it.
How about some insects or other invertebrates? I'm looking into getting some stick insects at the moment, they are really, really cheap and easy compared to fish and your DD will be able to pick them up if she wants to.
If you're dead set on the fish then I can give you loads of advice and links but have a good think on it, they are really not as cheap and easy as pet shops would have you believe!