A cheaper alternative to Chartreuse is Boomsma Claerkampster Cloosterbitter - it has 17 herbs instead of Chartreuse's 130, and it's noticeably less complex, but overall tastes quite similar.
I mix absinthe with moscato d'asti 1:4-5 like above. Moscato d'asti is much less expensive than champagne and if you're going to add sugar syrup to champagne you might as well have bought Moscato d'asti in the first place. I think I like the fresh fruity flavour of the moscato better with this too.