It sounds as if you have an implied agreement with the landlord that in exchange for giving you a break on the rent, he is leaving it up to you to either help out with any repairs or make the repairs yourself. IMHO, either you can have it one way or the other, but not both. $400 is unheard of for a home in the country in CA where you can keep chickens. If the septic tank needs replacement, it is likely because of lack of maintenance. Normally, it is the renters responsibility to have the septic tank pumped out. If this was not done regularly, (if you could not afford to do it) it could have caused the system to back up, which may be why it needs to be replaced now. I would rather not speculate but there are things you can and cannot do to a septic systems (some are really fussy) and it could be that you contributed to the problem without realizing it. At any rate, I think the repair problems should be shared equally even if you have legal recourse.