Tamara's suggestion regarding lease-options is good.
There are other ways to creatively purchase, as well. Another technique is called "subject to," where the property is deeded to you but the mortgage (for a while) remains in the name of the seller: It's a sale "subject to" the existing financing.
There are other variations: lease-purchase, contract for deed, etc. In those (as with lease-options) you start off by leasing and complete it by purchasing.
Sounds like the cart is now in front of the horse and you have already agreed to a price and are renting the property with a portion of your rent going toward the final price and your landlord is your banker. If that's the case, you've likely already signed an agreement or are operating on a verbal agreement, which isn't worth the paper it's not written on. It really doesn't matter what they paid for it, or what they want for it. The market value of the property is the price the Seller is willing to accept and the Buyer (you) is willing to pay. If you're in a position to back up and change your agreement, I suggest hiring a Realtor to provide you with local data of comparable sold and actively listed properties and represent you in renegotiations, get an appraisal and the advice of a real estate attorney. Yes, you'll need to spend money, but it's an investment and well worth it for your peace of mind. Your landlord is financing your home, but mortgage rates are at an all-time low right now. If your credit is good, you might do better by purchasing the property rather than having the sellers be your banker. Let this be a lesson for the future. Anytime you buy or sell real estate, it is a major financial decision. Don't do it without the professional advice of a Realtor, whose job it is to guide you through each step of the process, so you'll have the answers to these questions prior to coming to an agreement with the Seller. Good luck.... more