Uh, yes, I think you are very naive, but not about the price of lawyers.
That's a bit more than my divorce attorney cost me per hour, and that was 14 years ago, and I live in a medium- to low-cost city, so the $300 an hour probably is in the ballpark if you live in a bigger city like Seattle or Denver or Houston or Boston.
But if you are an unmarried couple, then any sort of contract between the two of you won't have any effect on any joint debts. Those will be governed by the contracts you signed with the respective lenders. Since they are not a party to any agreement between you and your partner, they are not bound by its terms. Most likely, any creditor has set it up so that each of you individually are responsible for the entire debt, and in case of default they will go after both of you, again regardless of any agreement the two of you have.
How joint assets would be handled probably would depend on how they are titled and what the rules are in the state in which you live, but again, I seriously doubt that any agreement you have would have any sort of legal force with regards to assets. In other words, if you jointly owned a house and one of you wanted to force a sale, you might be able to do that, but it would be based on your state law, and not on your agreement.
At best, any agreement you write up might forestall an argument later, or could be used as evidence in a court case, but if you're at that point the one of you has already sued the other, so the agreement probably didn't do any good.
And since any agreement you write up probably has no legal effect, then I don't see any reason to involve a lawyer. If I were in your shoes, I'd find a template cohabitation agreement out on the web, then modify it however both of you see fit, then both sign it.
IANAL, JMHO, YMMV.