I assume you've seen our warnings about the theoretical challenges of the Willingness to Pay concept. We generally recommend against doing this. Please see our white paper: http://www.sawtoothsoftware.com/downloadPDF.php?file=monetary.pdf
However, if you still feel you must compute WTP. And, if you want to do it from a single coefficient, then you can obtain a single price slope within our Latent Class procedure by clicking:
If using the Latent Class Standalone software, click the "Attribute Information tab". If using Latent Class built into the SSI Web system, click "Settings + Attribute Coding".
Then, select the attribute to be estimated as a single linear effect within the table.
Then, under the "Coding" column, click the dropdown control to indicate "Linear" rather than the default "Part Worth".
Now, in the panel just to the right, indicate the values that you want associated with each price level. Let's imagine your prices were $1.50, $2.50, and $3.50 (for three levels of price). You would type "1.5" in the $1.50 row, "2.5" in the $2.50 row, etc.
Now, when you run Latent Class, you'll obtain a single beta for price rather than three separate part-worth utilities for a 3-level price attribute.