even with the rocket booster and GPS guidance, you could get more than a dozen boosted smart shells for the cost of a single cruise missile.
currently the smaller M982 155m Excalibur rounds have a range of over 70km without rocket boosting, and possess inertial and GPS guidance for pinpoint accuracy.. and cost $68,000 per shot. a comparable missile (the 277mm M30 artillery rocket from the M270 MLRS) costs over $100,000 per rocket, and unlike the artillery shell it can't have the range boosted further.
most cruise missiles cost over a million dollars per missile, the US's tomahawks for example run $2 million per. so an artillery piece that can produce the same range and destructive potential at (potentially) a tenth the cost per shot was attractive.