A salesperson is responsible directly for a quota of sales. If they get that quota they get a big reward, and if they don't meet it they get less.
An SE does the parts of the sale that the salesperson is not able to. They articulate the technical fit, and summarize the business value of the solution. The show that it works in a PoC, and create a personal relationship with technical decision makers that is often unobtainable to the salesperson directly.
How can we quantify this? One way would be to check what percentage of deals can be closed without a specialised technical overlay, that is without a proof, demo, workshop or whatever the process calls for. For well established products this might be a higher number than unknown or new solutions. However for these it is a must have. Secondly, the SE might help sell additional services, find a need for additional product, or eliminate the opposition more effectively by laying "landmines" for competitive products.
A response by Phil Janus (www.salesengineering.com ) to Dave Stein's article What is a sales engineer worth? claims that:
Real customer data is showing 3 to 7 times more incremental revenue per year. SEs pay for themselves 2 to 5 times over annually. Annual program returns have ranged from 130x to 385x per dollar. One customer generated 678% more incremental revenue in 6 months. Stated another way, failing to implement a pre-sales initiative costs $300,000-$500,000 per SE per year in lost revenue.Certainly this shows that SEs are a great resource to include in the sales team. The next question is how many are needed and where best to put them. That of course will be in another post.