If you have NAV license, you need to order developer license from your partner. Cost of developer license depend of country region. License has different costs at different regionos. In my country (Serbia), cost is:
*NAV2009:
-Application Builder 5.200$ (+16%BREP)
-Solution Developer 18.200$ (+16%BREP)
-Developer's Toolkit - Source Analyzer 5.200$ (+16%BREP)
-Developer's Toolkit - Compare & Merge 1.300$ (+16%BREP)
*NAV2013:
-Application Builder 7.200$ (+16%BREP)
-Solution Developer 25.200$ (+16%BREP)