During software development, a major challenge is to estimate software development costs. Project managers' core objective is to deliver a high trustworthiness product within the budget.However, trustworthy software is related to several different attributes. Conflicts between different attributes may occur when a budget allocation scheme is adopted to improve trustworthy attributes. Therefore, it is necessary to select an optimal allocation scheme to reduce conflict cost. In this paper, we first propose a conflict cost and costs estimation model. The difficulty coefficient constrain for improving attributes are established. The importance weights of attributes are analyzed. Then, based on conflict cost, importance weights, and difficulty coefficient contrain, an algorithm to obtain an budget allocation scheme is presented, which can minimize conflict cost. Finally, an example is showed to demonstrate the feasibility of the proposed algorithm. The research can help the manager of software to use the budget funds more reasonably and gain maximum benefit.