In response to your question how to define a donor unit " Since mosdos have wildly varying budgets"
While it's true that mosdos have budgets ranging from 100 thousand (usually just starting) to several million (with many Educational institutions) and anywhere in between, usually several hundred thousand, that is exactly the reason that a unit is not a set number, but rather relative to what you need to raise, here are several examples. (this is very general, and must be suited to each place according to the realities in his situation)
You need to raise $180,000 then a unit is $1,800 and you need to raise 100 units (half a unit would be $900.00) you are looking to raise from 10 units and down. 1 donor at 10 units 1or2 donors at 5 and from here you work your way down and 1 unit will be the most you want to attract anywhere from 20 to
50 such donors. Also two half units makes up a full unit, so you may have potential donors not ready yet for a full unit but can get them in for half a unit.
You need to raise $360,000 Then a unit can still be $1,800 but now you will need to raise 200 units, or if your at a level where you've been around for a while and already have a good number of $3,600 donors and up then you can have a unit at $3,600 and half a unit at $1,800
If you need to raise half a million you can choose what amount your unit will be, it can be $5,000 if you're up to that level or $2,500 or even
$1,800 and round up the fundraising goal to $504,000 a total of 280 units, of course in this case there would have to be many donors at levels of 10 units 7 units 5 units and maybe one at 30 units a total of $54,000
Usually the one top level donor we strive to be somewhere in the 10% level of the entire fundraising goal.
BTW a Shliach just starting and needs to raise only $60,000 then a unit can be $1,000 to raise 60 units or $1,200 and raise 50 units.
The very important item to know is that this is NOT a gimmick to get a donor to give you money and more then he may have given otherwise, this is a chart derived AFTER working very hard at creating a true, honest budget and you really know what you need that need becomes your fundraising goal and then you divide it into units and then you go seeking the donors to help you reach your goal.