The optimization is essential for any high-performance querying system. Several optimization techniques were developed and successfully implemented for relational databases. However, these techniques should be re-examined and revised for distributed heterogeneous systems of information resources supporting diverse querying paradigms. We introduce cost models for approximate query evaluation in the context of generalized algebraic operations supporting both exact and similarity queries. The proposed cost models are suitable for approximate evaluation and trade-off between computational performance and the quality of results. We present a rationale for our approach and elaborate our cost model for key operations and algorithms.