It entirely depends on what you can do, and what you want to do.
If you pick up a run down house in a road where all other houses sell for 200k, and you get it for 150k. If it needs 50k of work, and you pay people to do it, you'll make very little, if it needs 50k or work, but 80% of it is done by you, and you only pay tradesman to do 10k or work, then you can make money, but ultimately a pretty large portion of the profits made by people flipping houses by buying run down, renovating and selling as finished is made by people working. Its a second job, and often hard work, plenty of people do a 9-5, then go and spend 6-12 at the house, and all weekend, for 3 months before they're finished, then make 10-20k.
It's a good second job, or a good full time job. The guys who tend to make a profit and NOT do the work themselves but pay for it usually make the money up in margins and volume. IE someone who does this full time and has a team of builders, so rather than wait weeks while paying a mortgage, he's got a full team. He buys a house, and has the guys there the next day. You have a couple labourers who clean up, while his electrician/plumber is working on another house, while the plasterer/painter/joiner is finishing off a 3rd house. Rotating guys, keeping every job going without any downtime, paying your own team and wasting no hours saves tonnes of cash and having several houses on the go at once means less overall profit per house, more houses.
Decide what you can put in before you go looking, be careful when you buy, auctions are crazy popular for the people that do this as auction is where a lot of the "worst" houses go which need the most work and have the most profit potential. If you overpay... you'll work hard for months on end and break even if you're lucky.
Make sure you have the money to pay for work, make sure you've found people who can do the work you need, or that you can do the work. A lot of where peoples profits go is having a mortgage, paying monthly while work drags on. You think it will only take 3 weeks, or 2 months, and hope it will sell within a month and only pay a few thou on a mortgage, but because you didn't find a builder first, had to look around for an electrician, had to wait for guy one to finish before calling in guy two and guy one took ages, you end up spending 6 months, spending 10k more on the mortgage than you wanted, and the profits disappear.
If you don't know buildings that well, take someone with you, a friend whose a builder(who won't see a HUGE problem, not tell you, then "find out" after you've bought that it needs a really expensive amount of work done that you'll ask him to do
), structural engineer, someone who can give a very good idea of the work doing, and spot major major issues before you buy.
