This is a question I honestly don't know the answer to, but much $$ are the Rams worth to the City? In other words, I assume that all the food/beer/merchandise/etc at Rams games brings in revenue that gets taxed. People come to the City to see the Rams play. They rent hotel rooms/eat/drink/gamble/buy gas/etc and all that gets taxed. I don't know about payroll taxes and all that, but I'd imagine the city gets a chunk of payroll as well? So...what is the economic impact of the Rams departing?
A complicated question. I believe the Rams have an exemption from the city entertainment tax (as do the Cardinals, the Blues pay in), so their ticket revenue isn't taxed by the city, and I'm not sure how that affects concessions and merchandise sales.
So then you are left with the ~60,000 fans that are downtown 8 times a year, plus a couple preseason games. I'd venture to guess that at least half of those are local (within the city/county), so you probably don't get that much extra boost from them (i.e., they'd be doing something else in the area that day anyway, eating, buying gas, etc.). We're down to the taxes collected from the ~30k remaining--and assuming that they only come for regular season games. Simple math, let's take 30k fans x $150 spent per fan x 8 games per year x 5% collected by local taxes = $1.8MM per year.
To break even on spending $700MM on renovations, you'd have to bring in $35MM in added revenue each year for 20 years (when they'll want another new stadium, if not before then). That's an average of ~$100 per fan. I don't think the impact of each fan is $100 every game.