But are slackbots actually made/sold and billed for amounts like that?
Google says median web developer salary is $65k/yr or $30/hr, and at $30/hr, $5,000 is 167 hours; or 4.1 weeks (at 40 hrs/wk), or roughly a month; now -> sept 30th, though that is crossed out in the article.
So $5,000 is within reason for that skill set, but do slackbots actually involve that much work? If you haven't seen Github's chatops original talk from four years ago it's worth watching, or at least reading the slides, along with Hubot; the product and its features/interface, as well as its code. There's a decent amount of code involved for Hubot, and Github paid for its development (over far longer than 4.1 weeks, mind you).
The pay is there and the work is involved enough to take that kind of time, but I can't answer if this particular offer will lead to any work, nor how involved that work would be, so whether it would involve stuffing a few strings into an existing framework, or a whole lot of new development; in turn making it a good or bad deal for the buyer, I also cannot say.
I was on a team that built a Slack bot in Powershell - she announced the number of incident tickets from each team's Zendesk queue, in the respective team's Slack channel. This helped drive awareness and SLAs.
Leveraging our team's in-house Powershell modules for Slack, Zendesk, and other internal Powershell tools meant the bot was only several hundred lines - not much more than a foreach with a config file.
The total depends entirely on the complexity of the request.
Unless you are a brand and can charge whatever the fuck you like for your work — things typically have a cost. You know how much a cup of coffee costs, a laptop, a car, a house. Chatbots and web-pages have their market price as well.
(And FWIW, I don't know, maybe it's ok kinda price in USA, but to me all this doesn't sound like a $5000 deal.)
You are thinking that people charge some amount derived from their costs. But it's more like people charge as much as they can (as the market allows) and the actual costs are only important as a lower bound - there is no higher bound.