It's not relatively complicated (I've built one for fun, though not production ready) but without knowing the full scope of the project he's wanting completed, it could get quite expensive. I can break down my logic a bit but honestly there are just a ton of variables that are at play.

Front-end: If you want a well designed front-end without an ultra basic bootstrap theme you're already looking at $500-$4000

Database: Sounds like he's going to need a database for all this information, you can basically tack on another $1000 - $3000 pretty easily.

a/b testing, data analysis, seo: These are all nice to haves (can launch without them anyway) but end up costing development time and energy.

Sys-admin / Security: Are you setting up and securing their stack or will they be doing so? How about ongoing maintenance and support? What about servers, are they planning on buying hardware or hosting in the cloud? Easily could add a few extra thousand here not including a possible recurring fee for lts.

Bot Code: Price can vary here a fair bit but you say, in addition to your code for less than $2000 but lets say you don't have something like this already created. You have to write a fair bit of code, it needs to be robust and well tested. Security is obviously going to be a concern if you're handling other peoples items (especially on a live site)

OAuth/payment processing:

You need to link accounts and if you're wanting people to pay for something on the site, that needs to be setup (ie, costs development time)

There's more that can be discussed but without knowing the scope of what Need_A_Bot needs, it could be super cheap or pretty gosh darn expensive. Maybe 20+ grand is slightly pushing it but it could surly go into the 10+ thousand range. Good developers are in high demand, as such the price for good work is high. Not to say that Need_A_Bot couldn't go with a cheap option and get something up and running, I'm just trying to help bring awareness to costs (this sub is damn near known for people trying to low-ball developers). I believe he was looking for something around $500 when he posted, that's absurdly low.