Real-world clinical competence demands more than answering medical knowledge questions—it requires proactively gathering incomplete patient information, selecting appropriate examinations, and formulating safe, cost-conscious treatment plans through multi-turn interaction. Existing medical LLM benchmarks rely predominantly on static QA formats, failing to assess interactive information acquisition, tool use, or cost–quality tradeoffs. We propose MedCPEnv, a reproducible, cost-aware simulated hospital environment for evaluating and training medical LLM agents. Built from MIMIC-IV-NOTE discharge notes, MedCPEnv comprises 48,803 curated cases, a 1,000-case benchmark, and a 371,550-entry medical QA knowledge base, exposing four deterministic action types (ASK, EXAM, KNOWLEDGE, FINAL) to eliminate high-variance LLM role-playing artifacts. Evaluating eight frontier models, GPT-5.4 achieves the highest composite score (3.69/5) and diagnostic accuracy (60.3%), yet treatment accuracy remains only 19.4%, identifying treatment plan generation as the primary bottleneck. Substantial inter-model variation in the cost–quality tradeoff further demonstrates that clinical correctness alone is insufficient to characterize agent behavior. GRPO-based Agentic RL on Qwen3-4B shows that medical quality rewards consistently improve tool-use strategies, raising the composite score from 2.97 to 3.14 and diagnostic accuracy from 26.0% to 31.2%, establishing MedCPEnv as both a rigorous evaluation benchmark and a reproducible training environment for medical agent research.