You ask how you can decipher the quotes you received for building a new website. What I'm wondering is what type of process these companies have walked through with you before delivering these quotes? Did you tell them what you need and then they gave you a price? Immediately eliminate those quotes. 

Discovery First
When you talk to a company about designing a website, the company you talk to should follow a discovery process before actually talking price. You might tell them you want X, X, and X features because you've done your homework and believe those features will solve your problem. Have any of these companies actually taken the time to discuss with you what goals you want to accomplish, what problems you need to solve, what metrics you want to reach? 

Solution Focused
Some companies only work with clients over a certain dollar budget. Other agencies, like ours, work within the client's budget to find solutions to meet their needs. 

Designing a website won't magically increase your sales and neither will getting on first page of Google. These things work to increase your visibility. 

The Right Quote
A well-crafted strategy with goals, implementation, and metrics is what's needed before starting any project. If any of the quotes you've received follow this path, those are the ones to consider. If none do, then you need to find an agency that is interested in helping you grow your business not just in designing a website.