Not cheap, but cheaper than knee-replacement surgery.
We've recently been converted to hiking with poles. I love the sense of 4wd I get and it inspires me to keep barrelling up switchbacks without getting fatigued.
The are almost impossible to use at the same time as an umbrella, but so far I've been pretty happy with them.
They are quite an investment, so you may want to borrow/rent a pair first to see if you like them.
Petes Pole Page, has some good info on using poles including a little tutorial.
Available from REI available for a whopping $199!
Someone should invent an umbrella attachment to thread onto the end of a trekking pole. Could either save the weight of the umbrella shaft or allow for a larger coverage area at the same weight.