The cost of a private on site sewerage system is dependent on many variables. The repsonses are spot on, but keep in mind, that the negotiations all hinge upon the buyer's willingness to assume the risk and the seller's calculation of value. There is a good potential to get a better price on the property as many buyers will not consider purchase with Title V issues and the potential for construction.
My advice - gain advantage through education and flexibility. I visit each town's Department of Health to gain knowledge of the code requirements. Many towns have a list of sewerage system engineers and installers (engineers and installers must be separate). The thought is that these professionals know their business and many are willing to share some ideas about the particular neighborhoods and comments on percolation tests in the past and soil composition (ledge, gravel, clay, water table etc) give a rough idea. Also keep in mind that the number of bedrooms dictate the size of the system - it may be worth while to install a four bedroom system, even if the property has three in the event of future expansion.
Other considerations - a mortgage lender that spells out the criteria tailored to your financial profile. 203K loans, rehab, may be available and considered. This essentially builds the repair or replacement costs into your loan. A real estate attorney, from my vanatage point, is needed before an offer is made to spell out the potential liability issues for you as the buyer and also the seller.
A bit of work, hence a Title V issue detracs many buyers giving you potential for better financial negotiations. Fully understanding the technical side is very important and with the right homework and advice from professionals will provide you with the risks associated with a major decision!
Final point, an agent willing to spend the time to help you map out a plan as a culmination of your research will help your negotiations - A well informed consumer with knowledge of the Title V requirements should make it more likely to introduce your offer in the best light possible.