You gain plenty of postivies with that expense. Individual energy independence, system resilency, price stabalization, on and on. Lifespan of solar panels is 30+ years, and payback is often in the 4-7 year range. The numbers work out in your favor for a long time.
But okay, leaving out rooftop solar, and just going grid scale. With the renewable mix your city has now, and assuimg your intial power usage numbers are correct, you need 30% of the 150km to have the city running on 100% renewables. Thats 45 kilometers of nearby land. Seems way more doable than 150km, especially if its 10km here, 15km there, etc. Using a mix of agrivoltaics (farm + panels) and distressed land (old mining/industrial/military/etc) should make that even more viable.

