I have a slightly different take.

Since you have 5 accounts now, why not take advantage of it? I will never level up a single toon again. I recently leveled up a set of four toons (being boosted by a 70 hunter up to 20 for run speed, then a prot pally in instances (Stockades/SM) to 40, then questing in Tanaris (with hunter killing everything for them) to 45, then boosting in Strat by my pally to 58, then boosting by hunter/mage in Outlands up to 70.

Having boosted 2 up to 70 before in a similiar fashion, I found that boosting 4 in this way took only marginally (5% - 10%) more time. With an ele shammy boosting in place of the hunter/pally it will take longer (you may have to forgo the instance boosting and just quest boost all the way to 70), but I would definitely boost four instead of one.

I guarantee that if you got your pally to 70 alone, pretty soon you'd wonder what it would be like to have a Boomkin join your team in place of a shammy, or a priest to heal instead of your resto shammy, or a warrior or druid to tank instead of your pally, or a hunter to solo in Kara just for fun, or.... etc.

Four for the 10% more than the price of one alone? I'll take that deal every time.