Here's how i built my forge for under $300! biggest cost will probably be your firebrick. This can be built cheaper but this whole project took me an hour doing it this way.
Supplies- angle iron, 1/8" flat steel, square tubing, firebrick, propane burner and regulator, knowledge
Soft firebrick-https://amzn.to/3sKsn7O
Burner and Regulator-https://amzn.to/3iuiUNn