Mrgman,
Considering your speakers it is not surprising that you find a quality matching sub prohibitively expensive. Low frequencies are that much harder to reproduce without excessive distortion (20% and much more at the higher SPL levels which your speakers are capable of). Consider that a lower quality sub or a mis-matched sub may cause you to lose some of the advantages of LF clarity, dynamics and balance that you get from your monitor speakers.
Considering your speakers it is not surprising that you find a quality matching sub prohibitively expensive. Low frequencies are that much harder to reproduce without excessive distortion (20% and much more at the higher SPL levels which your speakers are capable of). Consider that a lower quality sub or a mis-matched sub may cause you to lose some of the advantages of LF clarity, dynamics and balance that you get from your monitor speakers.