I find the ranges reported at this site to be mostly accurate --
https://www.fixr.com/costs/landscaping
Getting estimates from qualified local contractors who have done this sort of work for your neighbors is generally the best way to be sure that prices are inline with local rates. The best time to seed is generally fall or spring, honest landscapers will explain that seed germination during the hottest part of the summer is very tricky and even with lots of water you may see much of the seed eaten by birds and wildlife.