For outdoor banners, check out bannersonthecheap.com. A full color 3x8 banner is only $29.
For indoor banners and posters, have you considered purchasing a wide-format printer? Including the cost of paper, supplies, and maintenance, you can print a 3x8 indoor paper banner for $20. At Kinko's/FedEx, the cost of a 3x8 paper banner is $139. How's that for some savings by doing it on your own? There are a wide range of wide-format printers on the market. I suggest HP brand because I've had 13 years of experience with them and they never seem to break down. A high quality new HP wide format printer will run you $3500, but you can also find them refurbished online for as low as $1500. If you buy new, for example, you only have to print 30 posters at your cost to make up the difference between printing in-house vs. printing at Kinko's/FedEx.