Sure, if you have a half-rack you are going to get screwed in terms of power provisions; you aren't really paying for power. Likewise, if you go with a site that is designed for high density and you are a low density tenant (or the reverse), you are going to get screwed.
For a bigger tenant (every lease I have ever worked on at least), you generally negotiate a fixed PUE* of 1.4-1.6 to cover cooling and system losses. The annualized PUE is usually closer to 1.3-1.4 for most general-purpose sites at reasonable load factors, so they effectively have 14% markup. If you have a really good lease, that markup gets factored out based on actual operating costs.
*PUE(tm) = Power Utilization Effectiveness(tm) = Peak Critical Demand (kW) / Peak Total Demand (kW)