The opamps are powered with +/-15V, generating 3.3V seems complicated from that with a linear regulator... I don't know. The DAC consumes about 430mW.
It's not complicated, but with just an LDO, it won't be very efficient indeed. I don't know how much current you can get from the +15V for instance... Maybe the additional current that would be drawn by the DAC would exceed what your converter can provide. You could use a DC/DC converter (not isolated) instead of an LDO, to go from +15V to +3.3V. A lot more efficient. Still, your isolated DC/DC converter should be able to provide the additional 430mW, not sure it can.
Otherwise, yes, you can consider using an additional isolated DC/DC converter. This for instance: PQME1-S5-S3-M is 3.3V/660mW and 3.91€ per 1 (check for input voltage though - this one may not be a match, but I think you'll find ones that will in similar price range). I don't think it's expensive, but again I don't know what your budget is, so...
My DAC is dual SPI, one for data another one for setup, and a reset line, it's at least 7 inputs to isolate.
OK, what's the SPI clock? That would require two of those common digital isolators. We need to check whether they'd be able to handle your SPI clock speed though.
The Si digital isolators (quad versions) start at around 2€/per 1, it's not that expensive really. (AD and TI also have similar parts.)
You can take a look there:
https://www.silabs.com/products/isolation/digital-isolatorsSo all in all: 1 small isolated DC/DC + 2 digital isolators = about 8€ for very low quantities, significantly less for high quantities. Is that expensive for your application? Your call.