I could do it. Easily. I think my new large en suite (tots up quickly) under 600. The smaller en suite (with electric shower) cost a bit more because the cabling for the shower needed to be redone (300), I bought a new electric shower, tray and bifold doors for an awkward space (350) and I paid a suitably qualified person to come in and fit it (150). But I did most things myself and anyone who is the slightest bit handy and has a copy of the Readers Digest Guide to DIY could too.
So large en suite was a second hand (but lovely) acrylic slipper bath and beautiful mixer taps and pipe shrouds (150), a second hand massive chrome race track shower rail (40), an Ikea Hemnes washstand from bargain corner (60, but they're not masses more new), a new Ikea sink to go on it (60), a nice tap from ebay (35), a loo (60) a few bits and bobs in terms of mirrors and mats (30), a chrome towel radiator from Screwfix and valves (40), pretty bathroom paint from Dulux (30), a lightfitting from Ikea (10), vinyl floor tiles (Gerflor Pop, 6 sqm + postage, bought 5 sqm, think it was about 40) and a couple of shower curtains. Done.
I personally don't like tiles very much, but they're not difficult to do if you go slowly, prepare the walls and make sure your lines are straight. And use tile spacers, a good tile cutter and a tile nipper/saw for awkward bits. What might be tricky would be if your walls need replastering to make good where the tiles were, which I don't think is a DIY job personally. Tiling is very expensive per square metre to have done, but you can buy some lovely tiles for around 10 a metre. I like metro tiles more than any other, which isn't saying much, but large tiles are good in bathrooms as there are fewer grout lines for water ingress. I think sheet vinyl is a neat finish and has the added advantage of being quite cheap. We were quoted 150 to do our 3m by 2.5m en suite (in a white vinyl from carpetright), but in the end I used vinyl tiles. Which were awesome by the way, I've bought them for my utility and all the other bathrooms since laying the first lot. The black and white ones look particularly nice in a chequerboard pattern in our big family bathroom with the cast iron bath.
Plumbing is easy. I don't touch electrics past replacing the odd light switch and fitting, but plumbing is just like lego. Especially with push fit piping and compression joints. I think copper compression is slightly more fool proof than pushfit, but I use both fairly happily. You just have to remember that you will always need to tighten stuff up a bit, check and double check for tiny leaks, never lose your washers (v important) and have someone else to hand to turn on the water again so you can check you don't have a massive leak straight away.#
This sounds like a big braggy post, but if you are on a tight budget, but have time and are committed to finding the best deals, you can do it. It's a bit of a slog sometimes (especially when you have a whole house to sort out) but for us, it's the only way it can happen. We literally cannot afford to give our builders any more money, so if stuff is getting fitted, we are doing it. Once you have the tools and the experience, you can do it all again too.