We have published CPP's for both of these models. The SP 4510sf is anywhere from $.03-$.035, & the MP 401spf is $.0155-$.017. Based on that gap, the break-even is about 41,000 images. On a 36 month lease, that's a little more than 1,100 images per month, whereas on a 60 month lease it's not quite 700 images per month.
Personally, I like the strategy behind having the two models priced differently. The printer manufacturers (specifically HP & Lexmark) have been doing this for years, & it gives you options. Typically, I would deploy the low-cost/high CPP device as the standard for single users and the high-cost/low CPP device as the standard for workgroups. That being said, there's no way in hell I'm going to be selling a $.030 per page device with any type of regularity.