Regarding cost - Keep in mind some older homes may cost more due to the EPA RRP lead paint rule that was placed in effect. I'm not saying price will be doubled, but it will add some more labor and material cost. After all, the EPA is fining non compliant contractors nearly $38,000 per non compliant violation. I see many posts about illegal contractors. We do pay insurances, we have business costs, I price jobs to where I am comfortable with paying labor, expenses for my business, and not making $250k per year as some do. I have gone into homes where someone thought they were getting a great deal and ended up calling me to fix mistakes or sloppy jobs. Paint will vary in cost, yes.
Entire interior house painting job (excluding ceilings). 3 Bedroom / 2 Bath, Kitchen, Dining Room, Family Room, Living Room and Utility Room. Nothing really out of the ordinary, single level, no unusual architecture. I got estimates from professional painting companies, both small and large and individual "painters for hire". What surprised me was the huge range of estimates. $2,800 on the low end to $10,500!! on the high end. Finally went with a small family company at $4,500, (the lower bids seemed a little disorganized and unprofessional). The company I went with may not have been the lowest bid, but they seemed to know their stuff, spending over two hours doing color matching and comparisons just for the estimate. I couldn't be happier with the result, all their suggestion's were spot on, the house looks amazing!
Don't hesitate to ask all the questions you need to feel comfortable hiring a professional. You can also ask 3-4 professionals to make sure you have the right quote in mind. You must make sure they have a warranty or some kind of paperwork stipulating the quality of their jobs. Insurance is also a #1 priority to consider, as damage can happen in the process of painting.
The price to paint an interior versus an exterior depends on a few factors. For painting an exterior, you will pay about $2,600. This is because you have to account for the siding, making sure the weather doesn't affect the job and dealing with weather conditions. On the other hand, the price to paint a home interior is around $1,700 -- less to worry about other than keeping the flooring and furniture paint-free. If you want to do both the interior and exterior, talk to the painters about any deals or their opinion on timeline and how it affects your budget.
Whether doing it yourself or hiring someone, buy the best quality paint you can afford; it lasts longer and in some situations requires only one coat of paint instead of two. Painting a room usually involves prep work, primer and latex or alkyd paints with a range of finishes. HGTV.com gives a quick overview  as well as a list of painting dos and don'ts .