I know its very less used TLD so worth is almost zero.
.pl is 16 most popular domain extension in the world and 11 in the ranking of country domains, so it's not very less used.
USA.pl can be worth a lot of money (min $xxx.xxx). In the USA living 10m people of Polish origin and Polish peoples have a fondness for USA. So I think this is not good example. Most if not all keyword domains that can be worth something are registered.
Here some sales from 2015-2016 of english words(disclosed), maybe it will help you decide:
SocialMedia.pl +-$1300
Powerbank.pl +-$1200
Concrete.pl +-$800
LiveCasinos.pl +-$700
Computer.pl +-$420
RealEstates.pl +-$260
MyFashion.pl +-$236
Flying.pl +-$220
Executive.pl +-$220
YouTbe.pl +-$220
Soap.pl +-$200
Enjoy.pl +-$200
And examples of older (there are many more in the $x.xxx - $xx.xxx range)
play.pl +-$530.000 (Play is one of the largest mobile network in Poland)
l.pl +-$22.000 (2012)
material.pl +-21.000 (2010)
domain.pl +-$26.000 (2008)
sms.pl +- $19.000(2013)
ss.pl +- $19.000(2008)
next.pl +-$15.800
broker.pl +-$13.000 (2010)
tools.pl +-$11.000 (2009)
startup.pl/start-up.pl +-$11.000 (2010)
bb.pl +-$9000 (2015)
speedway.pl +-$8000
oled.pl +-$8000 (2009)
halloween.pl +-$5500
airtickets.pl +-$5000 (2010)
natural.pl +-$5000 (2010)
youp*rn.pl +-$3600