There are many considerations. For example, buyer may capitalize the expense and amortize it sooner: on their schedule, not yours schedule of payments, and get it off tax base sooner. The budgets for R&D (your contracting rate) may be different than budget for software purchases. Depending ob legislation, they may get tax credits for R&D work (even though done by a contractor). They may have budget now, but are not sure about later, but still want software to work, no matter what