CloudShare is a great approach for a long term SharePoint development environment even with full and part time + temporary developers. From a Cost stand point, I would have to guess CloudShare would save you a ton of money and time. CloudShare offers full infrastructure maintenance and allows each user to setup their environment based on what they need. Additionally the CloudShare environment templates are hard to put a price on. These templates can help get a development environment up and going in no time at all. With CloudShare the hardware, hosting and licensing is all taken care of for you and there is no need to worry about setting up the servers and then installing the development applications along with installation of SharePoint. We use CloudShare every day for our development, training and even product demos and it has saved us countless amounts of hours and thousands of dollars. I would consider taking a deeper look into CloudShare and what it has to offer for your development needs and sign yourself up for a trail account. I can assure you that you will think highly of their services.
Hopefully the info helps! Cheers!!