So I think the easiest baseline for pricing is start with the cost at brand new, assume that used parts should be somewhere in the range of half that, then check some common forum and Ebay.
For the four barrel conversion, because you are looking at a manifold and carb, TheZStore sells the whole thing for around $900.
http://www.thezstore.com/page/TZS/CTGY/classic17a07b There is one with a used carb on ebay for a current bid of $300 and I see one posting on HybridZ for an Arizona Z Car unit (they sell the manifold only and slightly cheaper) asking $450 for the whole thing. This is the easiest option to tune due to the fact that there is only one set of jets and you aren't trying to get two carbs to work together properly. Great reliability due to new parts and you could do a manual choke or electric on the carb.
TheZStore has the Webber DGV carbs for $870 new.
http://www.thezstore.com/page/TZS/PROD/classic17a07c/11-3005 The page says 240z and 260z, but these will work on the 280z, we would just be removing the whole EFI as a unit. This is the setup I have on my OEM motor and it should work really well on an L28. I have jets for tuning and they only saw a season of use since I cleaned them. Even before I cleaned them, I had no reliabilty issues. I have started that motor in the middle of winter with only a little bit of effort, but that's says more about the health of the motor than the carbs.
A quick look on Ebay shows one used set for $445 and another for $259. I see one posting on HybridZ from 2011 that looks like he sold a good set in the neighborhood of $275.
The OEM L24/L26 SU side draft carbs can make decent power and could be used as a viable solution. I think that it's the vintage racers that usually go the route of hotrodding the old SU's. They are required to use the OEM stuff and you aren't. In order to get the the same power levels, there are much better routes, IE any of the other options. I have a set of three screw round tops, but they are in questionable condition, would need a good cleaning, are missing some parts (such as choke cable) and I'm not as confident in tuning them as I would be the DGV's. A reasonable price seems to be between $100 and $200.
I didn't mention doing a set of triples because you are now looking in the range of $1000 just to aquire a set. Tuning is much more of a nightmare, especially because you already have a big cam, but this is the true route to big power out of an L28.
Swapping heads or sourcing an OEM cam is not the way I would go. The E88 head that I have alone is not going to raise the compression as I had been thinking, you have to swap from dished pistons to flat top pistons. The valve chambers are the same. It would be a lot of work and you would be basically throwing away all the time and money someone put into doing the hot cam. Doing a carb conversion is much simpler and cheaper. If money wasn't an object, I would do the four barrel. I think they are cool, they have a good sound, run real good and would be super reliable. Of the two options I actually have available to you, the DGV's are the way to go. I have all the parts to make them work, I've had them off my motor before, I've cleaned and tuned them. I think I even have an intake/exhaust gasket downstairs (if I don't Napa has them readily available). I also have the Crane MSD box, that isn't working properly with the L28ET in my car currently, and we could get that (and the optical ignition) running on your motor as well. I could definitely feel the difference without it working.