I have no idea whether N73 production will stop or not but even if it does the present owners of the phone have nothing to worry about. The phone will continue to get support from Nokia in case there is some problem with it. Nokia has also just launched a firmware upgrade for N73, so that's not a problem as well. Plus N73 is a very popular phone. After the production stops (if it ever does), the demand for second hand phones will increase. So if you've bought a N73 now you'll get a very good price if you decide to sell it later on. N73 itself is a very good phone with an excellent camera, good display, fantastic loudspeakers and great features including Symbian OS.
N78 might be the replacement for N73 but that does not necessarily mean that Nokia will stop the production of N73. N73 was also supposedly the replacement for N70 but N70 continued to sell even after the arrival of N73. As for the price itself, there is absolutely no way that the price of N78 will be higher than the price of N82/N95. If my guess is correct, it'll cost around 17-18k initially and drop down to 16k in a months time. Going for N78 instead of N73 makes sense, but only if the budget allows. Otherwise N73 is a good phone on its own.