When is commercial space travel going to be common and relatively cheap?

When there is a place to go, and a place to stay that people want to go to. There's nothing out there now.

Probably not until after both of our ice caps melt.

Roughly 50 years or so.

When fusion power generates more energy than it costs. At that time, intersteller travel will be possible economically. With chemical fuels, space travel is prohibitavely expensive.

When a new propulsion system is developed that will drastically lower the cost per pound into orbit. The cost per pound is not only about the dollar amount but the amount of fuel needed to lift one pound of cargo into space.
Real rocket scientists help me out here

Given the cost to design and build a suitable space vehicle,
stock it with consumeables, fuel, liquid oxygen, etc., and
equip all occupants with custom space suits at 4 Million
Dollars each, and put each traveler through millions of dollars
worth of extensive training and medical testing prior to the
flight itself, my guess is that you will not see anything like
this occur in your lifetime, or the lifetime of your children.
At this time there is little they would wish to go and visit anyhow,
except go for a pleasure ride up and back down. The prospect
of being trapped in a tiny little capsule for 30 - 60 - 90 - 120
days is not to exciting. Eating everything out of plastic bags,
and doing your thing in a special container gets old rather
quickly one can imagine. It really seems much more desireable
to go fishing in Hawaii, or skiing in Canada to most people I think.

As far as going to the various stars on a commercial flight..., well, spend 8 bucks and purchase another science fiction book to read. That will not happen for several thousand years, if even
then. You actually wouldn't want to visit the star out there because it is quite similar to our Sun, and you would be fried to a crisp. What you might dream about doing would be to visit one of the planets of a distant star that might be the equivalent of Earth, because our selection of planets in this Solar System does not seem to provide an a planetary equivalent to Earth.

Not soon enough. I have been waiting for it for about 40 years now and I am running out of time!

It will be common in the next 50 years. It will never be cheap.

Testing of the first commercial (suborbital) spaceraft is scheduled to begin this year, with the first commercial flights in 2008. It is the full scale version of Bert Rutan's "Spaceship One" that made the headlines a couple of years ago as the first commercial spacecraft to actually make into space. Tha new ship is to be owned and operated by a new company--Richard Branson's "Virgin Galactic. This first ship is to be named the Enterprise (really!).

Getting from this to commercial space travel into orbit-at a (relatively) low cost is going to be a big step. Currently another company--SpaceX--is among several working on spacecraft and launch vehicles to do this. In addition, Bigelow Aerospace has (using a converted Russian ICBM) orbited a prototype satellite (Genesis 1) to test new technology for low cost habitats for use in commercial operations.

The timetable is uncertain--but given what's going on, I think that we can look for the first commercial orbital flights and passengers by the end of the next decade. At that stage its still going to be very expensive (probably $500,000 or more for a "tourist" flight (thought compared to teh $20 million that space tourists have been paying, its obviously going to be a great improvement.

Once that base is in place (i.e., a viable if small commercial space industry) we can expect costs to drop over time. There's no real way of telling how fast, though. But there's a very good chance that those costs will falll pretty rapidly. The reason is that technologies in their early stages (as this will be) are usually very innovative--and that willl translate into reduced costs. The other reason is that the various firms involved will have terrific incentives to bring down costs as fast as they can. This is a result of how the expanding markets of new technologies work. At first, such technologies (other examples include computers and airplanes) are very expensive--and few people can afford them. But the number of people who can afford the new technology grows faster than the price falls--so the market--and so potential profit--increases as prices drop.

I'm giving you a flat-out guess here--but I wouldn't be surprised if you're able to purchase a round trip ticket to visit a space station for under $50,000 by 2035.

when newer more cheaper reusable energy efficient propulsion system for rockets become available, and private companies venturing into the space industry become commonplace

Air travel is still relatively expensive.
I think we are quite a ways off.

Only time can tell