You can get a small STM32 with multiple multiplexed 12bit ADC channels, with ~2MSPS sampling speed and 256x hardware oversampling (~16bit), for less than half a buck (per 1000)[1].
Cheapest independent 12bit ADC with >500kSPS I could find on DigiKey were all over a buck (per 1000)[2], and they were all single-channel parts.
Now keep in mind that the ADC only does ADC, while the STM32G030 has several 16bit hardware timers and other peripherals.
Kinda hard to compare, since the external ADC doesn't do much without speaking to a microcontroller...
The STMG030 running full tilt at 64MHz, with ADC and one timer (4 channels) enabled takes about 6.7mA at 3.3V according to the simulator.
However looking at consumption in the sim with and without the ADC enabled at full speed, the ADC peripheral itself seems to consume a bit over 100uA. In comparison the ADS7042 datasheet says about 200uA at 1MSPS.
I haven't hooked up the STM32G030 (just got some), so I can't verify the simulator. Would be a fun project though.
Of course the STM32 has power-saving sleep modes etc, which reduces the overall chip consumption to a few uA.