I heard somewhere (author site *shrug*) that it's 250 as a rule. I hope that helps.
That's manuscript pages (in standard format), not printed book pages.
Roughly speaking, a standard adult hardback will have about 350-400 words per page, but typesetters can increase that or decrease that quite easily. Also, bear in mind that's *taking up the space of 350 or so words*, not 350 actual words, as this will vary accoridng to paragraph lengths, the amount of dialogue. the margins and other white space. It's down to the publisher in the end - do they want to make the book look longer (and increase its perceived value for money) or do they want to save printing costs if they can compress it to a more economical wordcount. (One - adult, SF/F - publisher I heard speak recently said that 416pages was a good length for them (about 95000 words), so they had a spine that was thick enough to be noticed in the bookshop shelves.)
I once took two novels on holiday. One had much smaller print than the other, so though there was only a 50-page difference, the difference in word count was something like 80,000.
Children's/MG/YA books traditionally tend to be printed in larger typefaces than adult books (at least in the UK they are), so the wordcount per page will decrease accordingly.