Hegel,
Just purchased two, one for the concept bike and another for the Nourish Weslake Seeley F750 road racer for AHRMA.
5 Speed HD AMC gearbox with magnesium cases; 17 & 18T sprockets: NZ$ 4,800
5 Speed XHD AMC gearbox with magnesium cases & kick start; 19T & 20T sprockets NZ$ 5,300
Personaly, with the TTI gearboxes and other options out there, I would not mess with a Quaiff. It sounds like it would take a lifetime of learning about new Quaiffe parts versus old Quaiffe parts, what works and what does not. I have been running a five speed and six speed in the two race Seeleys; basically install them and more or less forget about them and move onto the next weak link. Gear change with the TTI gearbox is crisp, clean and precise. I have yet to miss a shift on these boxes.
When comparing a Quaiffe to a TTI, one must ask what the price of failure is for a given application. If it is road going then perhaps it is a tow home or getting thrown from the bike and run over. On a track there are costs of travel and accomodations, track fees, missed opportunities for track time and podium time as well as getting thrown off and run over. For the reliability, the price is a bargain. I have yet to hear of a TTI gearbox failure (other than one where the owner neglected to fill the box with lubricant).