Washington State laws does state that no person may drive a motor vehicle upon a highway in this state without first obtaining a valid driver's license.

Revised Code of Washington (RCW) 46.20.015 speaks in regards to driving without a license when it is a traffic infraction (compare to section 46.20.005 when it is considered a misdemeanor). Under section 1 of RCW 46.20.015 it states that it is a traffic infraction and not a misdemeanor under if a person:

(a) Drives any motor vehicle upon a highway in this state without a valid driver's license issued to Washington residents under this chapter in his or her possession;

(b) Provides the citing officer with an expired driver's license or other valid identifying documentation under RCW 46.20.035 at the time of the stop; and

(c) Is not driving while suspended or revoked in violation of RCW 46.020.342 or 46.20.345.

Section 2 of this code states the penalty for driving with an expired license, or any of the other conditions listed above. Here it states that a person who violates this section is subject to a penalty of two hundred fifty dollars. If the person appears in person before the court or submits by mail written proof that he or she obtained a valid license after being cited, the court shall reduce the penalty to fifty dollars.

Depending upon the amount of time your Washington State driver’s license has been expired you may be able to go to the WA Department of Licensing (DOL) and renew the expired license. According to the DOL the fee to renew a driver license is $25. If it is more than 60 days after your license expired, you will be charged a $10 late fee. 

It would appear from this information that your citation for driving with an expired license in Washington State is normally a $250 fine but that it can be reduced to $50 if you obtain a valid license (renewed license) and submit the proof of this to the court. You should be able to double check that this would be possible in your situation by contacting the court listed on your traffic ticket.