Occasionally I have ideas I think might be able to become a successful business and make lots of money which could be then donated to effective charities; however I'm not going to pursue these ideas but maybe if we all share ideas like this other utils will be able to pursue good ideas we have.

Anyway last night I was wondering if there's a possibility to start a business in something that is very expensive (eg, lawyers?) but charge really low prices for good lawyers and then get loads of business = loads of money.

Plenty of reasons why that wouldn't work, but maybe some why it could

Do ye have any such ideas?

My other one's include robot DJs

One big question I have is, Should I try to create a startup that does a lot of good, or one that makes a lot of money?

If a startup does something good but relatively unimportant (e.g. provides after-school care for children in poor families—although this may actually be harmful: look up "poor meat eater problem"), it is probably not worthwhile. So the question is, can I create a startup with a reasonably high probability of doing good in the world such that the good outweighs the good from the monetary benefit I would receive if I focused solely on profits?

I think the only startups capable of doing that much good would have to increase utility as a side effect of its main focus. For example, Yelp doesn't directly relate to alleviating animal suffering, but it makes it easier for vegetarians and vegans to find animal-friendly restaurants, thus making it easier to live as a veg*n. But I don't know if this is enough good to justify the potential sacrifice in profits.

Furthermore, it is not easy to predict how much profit a startup will make. Given a choice between a utility-increasing startup and a "selfish" startup, it may be impossible to tell which will earn more money.

People already try to make their businesses ethical, and they likely do it too much, so it seems likely that you should focus on the more unethical ones. This has been discussed in Choosing an unethical career.

What about superhydrophobic clothes? especially for outdoors-y people I guess these people are already kinda onto this, but I don't see anything for mountaineers.

http://www.youtube.com/watch?v=7is6r6zXFDc

Charging less won't ordinarily cause you to make more money. Roughly, prices are already set for maximum profitability according to the principles of supply and demand.

Also, there's no plausible reason why any of us would be able to think of a more optimal way for selling expensive products like legal services. We have no competitive advantage in that realm of thinking, compared to - say - economists. Before we read any economics, it would be unfair to expect ourselves to revolutionise the principles of pricing goods and services.

If you want to look into business ideas, I recommend looking at kickstarter.com and similar crowdfunding sites, get involved with local startup meetup groups (of course, you should aim to be able to offer some combination of expertise, funding and managerial ability), read about the idea of a 'minimum viable product', historical examples of successful startups, etc etc.