Unfortunately without having a chance to take a look at the preamplifier it would be difficult for us to make a fair estimate and therefore doing so would be a disservice to you.
That being said, our bench rate for repairs is $110/hr + parts and the average repair takes between 1 – 2.5 hours depending on the issues at hand; some repairs could be in excess of that. So as you can see we cannot say with all certainly whether repairing your preamp would be less or more expensive than purchasing a new preamplifier. It is understandable as well as prudent that you would consider this when deciding upon which course of action to take. Along those lines some people may find comfort in knowing that a new preamplifier is in fact, new, and less likely to have other issues down the not so distant road and find value in that.
Ultimately the choice is yours and at the end of either path your Rhodes will be singing once again and we are sure you will be satisfied.