Funny thing is, I enjoy tracking the price of flights. So, even after purchase I'll still check and keep track and I'm usually following United out of RIC, IAD & EWR to LHR.
Bought a ticket in early April at $1200 for Premium Plus on United for late October and early November. Flight is IAD to LHR over and then LHR->IAD->RIC back home. The multi city was cheaper than either RIC to LHR or IAD to LHR.
At the time of purchase, Basic Economy was about $700, Economy was about $850, PP was $1200 and Polaris was about $3900.
At this moment, a month since my purchase the same flight for late October/early November is -- Basic Economy at $950, Economy is $1100, PP is $1580 and Polaris is $3090.
The three flights from IAD to LHR have been tracking similar prices, except now the late night flight is more expensive than the morning and evening flights-- late night flight is BE $1100, E $1270, PP $1850 & Polaris $3090.