Another Diplexer design by OK1DNZ is at http://www.w1npp.org/events/2010/2010-f~1/ATV/MODULA~1/VHFUHF~1.PDF

It is rated for 100 Watts with 70 dB isolation. The special coax cable "Sucoform 141 Cu" is available from Mouser for about $12 per meter, which is comparable to the price of the discrete components in the other design.

Dan Schultz N8FGV