The relays are actually in the $1.5-2 ballpark--basically the first small enough, nothing special specimen i ran into for the sake of getting the prototype up and running. It is going to be a one-off instrument, so nre and materials cost is not a limiting factor.
I had the following requirements for the switches:
1. Vds>250V
2. Rds_on < 200mohm (for adequate resistance measurement accuracy)
3.Beefy enough to withstand dead shorts at 160V and <3us in duration. This one is a very fluid requirement and in reality it may never see a short-circuited load, but still...
The above leaves me with sizable transistors the parasitics of which cannot be neglected. Coss~200pf, that's per transistor. There's 200 of those in parallel (ultimately they have to be 400), where the drain of each one is hooked to a 100 ohm or less resistor...so this sounds pretty horrible.
The pulse itself has to be as narrow as 70-80ns and its rise time is maybe 20ns or so, which means that i don't have much room for slew rate control...
Also, there will be some charging/discharging current flowing thru all 199 resistors that are supposed to be off while one is on and is being pulsed. The effect of such a current on the final value of an already trimmed resistor is unknown; it may drift up/down or simply fail. There is a very thin line between trimming and burning. Once it is crossed, the entire component is scrapped.