The Collaborative International Dictionary of English v.0.48:

Inappreciable \In`ap*pre"ci*a*ble\, a. [Pref. in- not +
   appreciable: cf. F. inappr['e]ciable.]
   Not appreciable; too small to be perceived; incapable of
   being duly valued or estimated. --Hallam.
   [1913 Webster]

WordNet (r) 3.0 (2006):

inappreciable
    adj 1: too small to make a significant difference;
           "inappreciable fluctuations in temperature"

Moby Thesaurus II by Grady Ward, 1.0:

53 Moby Thesaurus words for "inappreciable":
   atomic, back-burner, corpuscular, dinky, dispensable, embryonic,
   evanescent, germinal, granular, immaterial, impalpable,
   imperceptible, imponderable, inadequate, inconsequential,
   inconsiderable, indiscernible, inessential, inferior,
   infinitesimal, insensible, insignificant, insufficient, intangible,
   invisible, irrelevant, little, microcosmic, microscopic, minor,
   minute, molecular, negligible, nonessential, not vital, petit,
   scanty, scrimpy, skimpy, small, subatomic, technical, tenuous,
   thin, ultramicroscopic, unapparent, unappreciable, unessential,
   unimpressive, unnoteworthy, unobservable, unperceivable,
   unseeable