If cost is an issue, then I don't think another pet PERIOD is a good idea. My two "free" cats cost me almost $5000 in vet bills in a 3 year period. As a student. On student assistance.
Pets cost money. Why don't people recognize that?
Anyways, about your dog being bored, there are lots of FREE things you can do to occupy him. Training is fun for both of you, and then you can entertain others later with the cool tricks you've taught him. Go on more walks, and vary your routes. Take him to a local dog park and play. And the list could go on and on.