Assuming a 4" thick sidewalk, and 5-6" thick driveway, you will need close to two yards of concrete, which accounts for several hundred dollars, plus they'll have to break out the old, haul it away, prep the ground, set the forms, pour, wait for it to kick, and finish it, so if you run the numbers, I would not be surprised at a $1500 estimate. Nevertheless, yes, to me, that is a lot of money, which means that you will want to spend it wisely, so as Ron said, and this applies to any project for which you will hire a contractor, always try to get at least 3 estimates, and, again as Ron said, compare them before you make a decision. It is not uncommon, as an example, for an $1800 estimate to beat a $1500 estimate, once you have a chance to compare the persons who will do the work, and what they do or do not include. On a separate nore, you did not mention the reason that you are replacing these slabs. If they are cracked, you are on the right track. If they are still intact, but simply settled below where they should be, you may want to consider mudjacking, which is essentially pumping a slurry under the existing slabs. I have seen excellent results with this type of repair.