Some deals are too good to miss. Others are too bad to consider. A deal Miami-Dade is to weigh this week to sell vital land falls into the latter category.

Unfortunately, this very bad deal is not only under consideration, but Mayor Daniella Levine Cava faces a commission order to rush to negotiate it because its backer says it’s too good for economic development to miss.

We’ve written before about this offer for county land to build a Costco and we called it complex. But now that the mayor is reporting more details to commissioners this week, it’s clear that this pet deal of Commissioner Kionne McGhee should be a non-starter.

In a nutshell, developers of a Costco want 17 acres in the West Perrine Redevelopment Area that the county bought for water and sewer operations. Mr. McGhee says the district needs Costco’s jobs and a deal is vital now. But county rules say land buyers first must find an equally good replacement site for county use.

Those facts came to commissioners April 18, when they ordered the mayor to hasten a deal and make sure there were no barriers to selling the land to would-be developer SG Cutler Bay Ltd. But the mayor’s report this week fills in a ton of blanks, painting a picture that is not pretty.

Most laughable is the developer’s offer.

The county bought land high above flood levels for $3.1 million two decades ago for water and sewer needs. Under a federal consent decree, the county must rush water and sewer upgrades, and this site that could operate in the event of flooding is a piece of that costly puzzle.

The vital site has been assessed for years at $7.55 million. But under commission orders, the mayor got appraisals from two respected firms that came in at $30 million and $32 million.

The developer is offering $4.5 million, and reducing even that minuscule amount by the cost of any needed environmental fixes.

If someone asked to buy your prized land, offered you 15% of market value and said he might have to cut that down if he had to spend money to upgrade the site, what would you say? That is, what would you say if you could stop laughing?

Well, that’s what the county should be saying now. Negotiations over.

Commissioners discussed the issue of price before these details were known and staked out their positions on what the county should get if it was able to find somewhere equally good for its water and sewer needs.

“If we’re going to poach county land,” said Commissioner Raquel Regalado, “it shouldn’t be for someone who doesn’t want to pay fair market value. And that’s really what this is about.”

“If it’s not fair market value and we don’t have property that’s naturally suitable in the right elevations then we don’t have to do it,” said Chairman Oliver Gilbert III in arguing that the mayor should play through negotiations with the developers and see how they turn out.

Eileen Higgins also argued for negotiations but said she expected the county to get reasonably close to fair market value and added “I am absolutely going to want to see what’s happening with water and sewer.”

What would happen with water and sewer in a move elsewhere is equally ugly.

“One of the sites considered is the old South Dade landfill property,” Mayor Levine Cava wrote to commissioners, which would result in a 30% to 35% increase in cost to the county.

The developer also promoted a Homestead site that is outside of the Urban Development Boundary and the county is looking at how much extra that might cost for water and sewer operations – but should the county even consider using land where development might seriously impact the environment and flooding could knock out water and sewer operations?

The mayor also revealed that the county has been targeting a move of the South Dade Light Fleet operations to the West Perrine land that the developer wants. That move, she wrote, would “open up the South Dade Government Center for a mixed-use transit-oriented redevelopment” linked to the new South Dade Transitway that is to open in less than a year. Such use should fit snugly into county plans for developing six rapid transit corridors that are in the works.

Even beyond the low-ball price offer, in other words, the facts would rule out selling land that the county wants, needs and has big plans for because a developer aims to build a big-box retail store. It should be a no-brainer “no.”

Unfortunately, it’s not. In the spirit of what the commission ordered her to do in April, Mayor Levine Cava has already sent the developer drafts of required economic development agreements to speed negotiations as the commission ordered. Tellingly, she wrote that plans for selling the site are “subject to quickly evolving updates.”

They should be evolving quickly into a firm commission vote of “no deal.”