Garden the size of a football pitch? The kinds of weeds that sink 1' down or more? The kinds of bushes that take a lot of ripping out? You should always get 3-5 quotes for any job - not estimates or you're asking for trouble :0)
I work for a management company and we hire landscapers all the time for commercial properties. And you're right, that sounds REALLY high. What size lawn are we talking about and how many bushes and what size?