I maintain the belief Sleipnir was put on the cash shop not to necessarily nickel and dime players, but to keep it from being ubiquitous. All the other primal ponies are essentially palette swaps with appropriate elemental design tweaks. Sleipnir, however, is unique. Sure, money likely played a factor considering they would want something exclusive to be made available when they launched the cash shop, but I very much doubt it was their only reason.