Okay, maybe not under $300, but I've been able to break down the build steps so each step allows me a few weeks to build the next phase.
I'm new to chickens myself and have had crash course in chicken math in the past few weeks. You will be surprised by both how much and how little space young chicks need. I bought a brooder that said is was good for 15 chicks . . .it failed to mention they would outgrow it within a week . . . and I only have 5.
My 5, 3 week old chicks are still indoors in this run/pen
https://www.amazon.com/dp/B00859JJPA?ref_=ppx_hzsearch_conn_dt_b_fed_asin_title_2 it's still too cold for them at night to be outside (or maybe I just don't want them to grow up yet

). It's plenty of room with feeder, waterer, roosts and brood light (I'll get pictures in the morning, they're sleeping now).
I was about to go all out and get them a large raised coop and enclosure, but realized they are still too small to reach the roost bars and such, so I decided to add a transition phase . . .
As the weather warms up I'm getting a second of those pens and attaching it to this dog house
https://www.amazon.com/dp/B000OLYD62?ref_=ppx_hzsearch_conn_dt_b_fed_asin_title_2&th=1 so I can transition them outdoors, but I'll still have a pen inside in case the weather turns nasty before they're fully feathered.
So far we're at $230, but I've been able to pay this amount over several weeks, $70 for the first pen, then $160 of the second pen and dog house. This makes things more manageable for my finances rather than trying to do everything at once.
Once they're fully feathered in another 3 weeks or so, I plan to get something like this coop:
https://www.amazon.com/Aivituvin-Ex...804293&s=lawn-garden&sr=1-26&ts_id=4619367011
enclosed in something like this:
https://www.amazon.com/HITTITE-Chicken-Outdoor-Waterproof-Anti-UV/dp/B0C99CFCMB/ref=sr_1_7?c=ts&dib=eyJ2IjoiMSJ9.YWr6wLPkMAIuVYWddj-z9ie9fqgQORBOAUsXzTmPgGwFc0emoRqMPW_zDGAH4O-pvovY276X7x-VNgUkqQU4bmmx0a-THGTJsHDmSrwJEDntcKLEeTHf7yM0glAgYrVmM1Pi0TwO47v8WnLzibWhyfVI26VBY8FRTLZ00Rey1Estt8K4XxO3aDzb_fwr1EVUQ6UA1OapfuBNvWJmJfkL4Zbq28UWJxnGQ-8p63VolVHHv8PrTCAoTjnKfe7rrT6Ik10zjXP2Tuv30vf5rEFM-F0QxC9Xrt0BULlNPy4aiphSV2V0b28QjGsL4_DW9D0Nh5HC_peTKbViS7O7HHxZ8qeMOxLfqw0jf9DyVUI6ZtA._o8EMrbxqOit_Ld3ekF6XCcGKnHOkB0E4MMBcIlV0Ws&dib_tag=se&keywords=Poultry+Habitat+Supplies&qid=1740804369&s=lawn-garden&sr=1-7&ts_id=4619367011&th=1.
If you're free ranging you can skip the enclosure. Also if you do get an enclosure be sure to cover it in hardware cloth. I almost ordered one that only had bird netting, which MIGHT keep the chickens in, but won't keep predators out.
This plan may not work for your situation, but it's made things more financially manageable for me and my family.
I forgot to mention, I plan to keep both pens and the attached dog house in case I need to quarantine new chickens, or a spot to put a sick or injured bird to heal.