Template logos are logos that are created beforehand and offered for sale. The same logos are sold to different companies.
Template logos, as name suggests, are not exclusive. When you buy a template logo, the idea is not that you use it "as-is". You could, but then you will probably be sharing that logo with several other companies who also did not bother to customize the template.
Modifying the template is a must. If you do not and someone else successfully registers it as a trademark, you could land in trouble for using a logo that you paid for.
Only buy a template logo if you
(1) can and do modify it yourself or if you
(2) only intend using it for a very short time.
Do not buy a template logo and take it to a logo designer to modify. If you're going to hire a logo designer, better to let him or her work from scratch. Designers prefer that and you'll more than likely end up with a logo that's better than the template - not to mention unique.
If you are on a tight budget, you might want to look at pre-designed logos instead.
Very little. There are some free template logo sites out there. If you pay for a template logo, about $20 is realistic. Anything over $30 is too much.
99designs.com sells templates logos for $99. That is very, very expensive for a template - so much so that it might not be obvious to all their buyers that the logos are in fact resold. You have the option to buy exclusive rights at $298 - which is also steep. Check out our pre-designed logos section for alternatives before you buy a logo through 99Designs.