I went through the Caps' signings on Puckpedia. Their records are spotty as you get closer to the 05 lockout (they don't have Nylander on there for example even though they have other older signings), but from what they've got here are the Caps' biggest pure UFA signings in the salary cap era. I'm looking specifically at guys signed during the free agency period, so not counting pending UFAs who re-signed.
Matt Roy: 6x5.75M (34.5M total) in 2024
Darcy Kuemper: 5x5.25M (26.25M total) 2022
Matt Niskanen: 7x5.75M (40.25M total) in 2014
Brooks Orpik: 5x5.5M (27.5M total) in 2014
Michael Nylander: 4x4.875M (19.5M total) in 2007
One thing that struck me going through the list is that since Nylander, the Caps do not invest in UFA forwards. The highest cap hit they've given a UFA forward since then is the 2x3.25M contract they gave to Justin Williams in 2015, and the biggest total dollar amount is the 4x2.75M contract (11M total) they gave to Richard Panik in 2019. Really the only other "significant" UFA forward signings were Mike Knuble's 2x2.8M in 2009 and Mikhail Grabovski's 1x3M in 2013.
I wonder how many teams have never signed a UFA forward to a cap hit above 5M or a UFA in general to a cap hit above 6M? Who knows how much of it's strategy and how much of it's lack of interest in the Caps from top UFAs, but that feels kinda crazy for a team that's consistently been successful and spent to the salary cap in the cap era.