Training my agents is expensive, because of frequent turnover, but not training them will have a huge negative impact on my customers. Is there a way to balance this out that, and is there a way of measuring my ROI on training?

Since my expertise is around Siebel, I will talk about that. Coming from the BPO industry attrition is a big big pain area for everyone. Siebel 8.0 has a concept of Task UI. For people familiar with Siebel, it is similar to the iHelp feature but is enabled with a workflow. What it effectively does is that on an inbound /outbound interaction, prompts the agent to go step by step getting the agent to fill out the relevant data in the relevant fields and moves from one screen to the next. It also allows you to start multiple tasks between which you can switch in the event that one interaction is over and another one started up before the agent could complete all the tasks on the previous interaction.
How this benefits you is the fact that the agents require little to no training on applications since the system will not allow the agent to go to a wrong field,and also ensuring that the business process defined gets followed properly.

Regards

Building and embedding a knowledgebase with symptoms and solutions based on the symptoms and product/service lines into your call center tools will allow for quick ramp up time, a metric important to have to get to ROI. Other great functionality is using skills-based routing tools that adhere to a tiered customer support model.

Siebel Call Center has a knowledge base built in that has this capability including call scripts (to GregY's point). Salesforce.com can be set up (custom only, though) to handle symptom to solution. RightNOW for the Call Center works really well as it is a complete call center too as it also EASILY incorporates live chat, a tool that is great for supporting world class customer support initiatives.

No matter what tool you use, it is incredibly important to have clearly defined, DOCUMENTED workflow processes and to keep training guides up to date. All too often I see clients put all the work into defining these processes, but not documenting them. Another big mistake is organizations fail to recognize it requires a dedicated resource to keep business process documentation and training materials current and deliver ongoing training.

Best of luck,
Melissa McCready
CRM Consultant

There are dialogs scripting and like tools embedded in Call Center and other CRM related software, you can use to promote desired behavior of your agents without heavy investment into formal training. The starting point of ROI measurement is estimation of a new customer acquisition cost for your business, length of your agents service, etc. That would allow you to assess risk of loss of a customer against cost of training.