Can I ramble on for a minute? I've got spare time today!
I think that your original question (or observation) of the relatively narrow spread between entry level engineers and project managers with 20 years experience is interesting and I've often wondered the same thing...For example, an entry level civil eng college grad (on the PE, project management track) may earn around $40,000 in my area. A project manager with PE and 20 years experience may earn roughly double that, about $80,000 (similar to what another poster said). In my opinion also, that's not a very wide spread between someone who knows nothing to someone who's very well seasoned.
Now, other things to consider are the following: the more experienced engineer may have been given the invitation of company ownership and with it, good size bonuses possibly. I know in our company, some bonuses for part owners are sometimes on the order of the entire year’s salary of an entry-level engineer. That's a little more like it, right? Or the experienced engineer may be the president/CEO/owner in which case their earnings would be higher too, I would think.
Although it's my humble opinion that engineering's a good way to make a living and it's rewarding in many ways, you're definitely not going to become rich doing it-it's just not the right vehicle to accomplish that in 99.9% of the cases. If you want to become rich and make real money, I think that you should own an engineering company and hire a CEO, managers, etc. to do the work for you. Or better yet, if I started a business, it probably wouldn't be in the engineering field at all. There's too much liability and not enough profit.
Furthermore, every employee a company hires has the potential to learn the profession and then start their own company. This has to be a sore spot for any engineering firm owner! I think the same thing goes for lots of chiropractors and doctors-who wants to train someone, show them the ropes, and tell them the secrets and then have them come back and become their competition!? In contrast, a widget manufacturer has employees who put together widgets. If there's no work, they get laid off and then when there is work, they come back happily. There's a much smaller chance that they'd break off and start a widget manufacturing plant of their own. See what I mean? Manufacturing widgets is the answer! It seems like arguments like these would dictate that the spread of salaries would be naturally wider between entry-level engineers and experienced engineers. It doesn’t seem to be the case though.