I just had a call with the people at Chetu, the developers I'm going to employ. They said they could get it done within five months for $30,000! I thought it was going to be like half a million dollars.
I'm not sure how reliable this review is but it seems like one of their clients had a terrible experience working with them[1]. I don't think they have a dedicated crypto portfolio either, CMIIW. What makes you choose them anyway? Do you have prior experience with them? I'm not sure how much it costs to develop a custom DEX so I can't comment much on the price they offer.
[1]
https://www.serchen.com/company/chetu-2/