Construction is more expensive because your throwing your money in to a bottomless pit. However with herblore you can actually make money whilst gaining xp from level 70 but you have to clean herbs.
Of course it's possible. The no.1 on rs : S_U_O_M_I is only less than a year from getting 200m xp in all skills. However thise will only be temporary due to the release of new skills next year.