1. Camping. You spend $50 once to buy the equipment, and then you can live almost anywhere basically for free. There are a lot of free camp sites https://freecampsites.net/ and cheap campsites in national parks ~10–20 per site. Besides camping is great bonding experience: both with nature and your partner(s).
2. Splitting cheap hotels with friends. If you are 4 or 6 — you can rent a 60 per night Days inn suite which would total 10–15 per person plus free breakfast. Sharing a bed with two friends could be a plus. Again, bonding experience.
3. Hitchhiking. Not so much in the US, though worked for me many times in time of need, but easily in the Northern Europe and South America. Saves you a lot of money and provides additional travel experience. Works best for a company of 2. By yourself — scary, more people — harder logistically.
4. Couchsurfing. Sometimes you do need a shower. Plus your host usually a unique guide to the place. Takes time and preparation but totally worth it.
5. Cooking. Regardless of the selected options above, cooking always saves a lot. Plus bonding experience, plus time to share stories with your host in case of the Couchsurfing. Always eat good breakfasts when leaving a place to skip the lunch. Use groceries rather than fast food places to save an extra buck and time (in case of Europe).