Nah. 5.7X28 ammo typically runs about $.50 a round, under $30 a box. Not to say that is not expensive but it ain't $50 a box. For comparison, .22 Magnum ammo (when you can find it) is about half the price ($.18-$.40/round).
Well now....this is news to me. If it comes down a lot more it may sell.
When I was first introduced to the 5.7s the FN Rep was trying to sell the ammo to me at $50 for 50 rounds. 250 rounds was "only" $235
I was teaching a class in Reno to Sheriff's officers and to some personnel from Reno PD, and the FN Rep came to show their new guns.
We all shot them and they were pretty cool, but WAY more expensive then the M4 carbines we were using and with ammo that was $1 a shot. Keep in mind too that the carbines were full auto. So at about $4 to $10 each time a cop pulled the trigger in the full auto mode, we all laughed at the notion that the county and the city were going to pay that much to train their cops.
He didn't get even one sale.
That was when the guns were "Law Enforcement Only" but he told us the pistols would be sold to civilians soon.
Nearly all the officers told him to show us what the 5.7 would do that their 9mm and 40s would not do.
He could only say "it holds more ammo" and "it's light weight".
At $1 a shot no one cared about it being light and holding a lot of ammo.
A Glock 17 with one extra mag has a lot of ammo too, but you can actually shoot the Glock if your last name is NOT Rockefeller. And at that time cops could by a Glock through the department for just a bot over $300 and M-4s were costing the department about $400
If the ammo is down to .4 now that's good news, but at close to the same level of performance as a 22 mag, it's still too high. (I think anyway....)