Just do it
It all depends how frugally you live.
It's possible to buy a bike for £500 or even $500, strap a tent, sleeping bag and stove to the back and hit the road, free camping and living on a few pounds a day for food and petrol.
On the other hand you can buy a new bike for £8000, spend £3000 kitting it out, then air freight it to somewhere on the planet for another £3000, spending £50 - £100 a day on hotels, meals and entertainment. Then there are visas, carnet, insurance, air flights and so on.
The main thing to do is work out how much you can afford and build a trip around that. £15 a day is a good guide for day to day living, but it's also easy to spend more or a lot less if you free camp.
The main thing is to just do it.