I built one with a feather board and BME680 (for weather and gas sensor). Still ~$150 with a few add-ons for a “plug and play” (plus programming) option. My neighbor has a Purple Air I can compare to and it comes out pretty close (after some tweaking).

1 x Flanged Weatherproof Enclosure With PG-7 Cable Glands[ID:3931] = $9.95

1 x Adafruit Feather M4 Express - Featuring ATSAMD51 (ATSAMD51 Cortex M4) [ID:3857] = $22.95

1 x Adafruit AirLift FeatherWing – ESP32 WiFi Co-Processor[ID:4264] = $12.95

1 x Adafruit PMSA003I Air Quality Breakout (STEMMA QT / Qwiic) [ID:4632] = $44.95

1 x Adafruit BME680 - Temperature, Humidity, Pressure and Gas Sensor (STEMMA QT) [ID:3660] = $18.95

1 x FeatherWing Doubler - Prototyping Add-on For All Feather Boards[ID:2890] = $7.50

1 x STEMMA QT / Qwiic JST SH 4-pin Cable - 100mm Long[ID:4210] = $0.95

1 x STEMMA QT / Qwiic JST SH 4-pin to Premium Male Headers Cable (150mm Long) [ID:4209] = $0.95

1 x Stacking Headers for Feather - 12-pin and 16-pin female headers[ID:2830] = $1.25

1 x USB A/Micro Cable - 2m[ID:2185] = $4.95

1 x Lithium Ion Polymer Battery Ideal For Feathers - 3.7V 400mAh[ID:3898] = $6.95

1 x STEMMA QT / Qwiic JST SH 4-Pin Cable (50mm Long) [ID:4399] = $0.95

1 x Adafruit Perma-Proto Half-sized Breadboard PCB - Single[ID:1609] = $0.00 —————————————————— Sub-Total: $133.25

Edited to fix weird type size and spacing on the order I copy/pasted.