Objective: To analyze and compare the radiation dose and image quality of kilo-voltage cone beam CT systems on different Varian accelerator platforms, providing data to support clinical decisions on selecting optimal protocols for image-guided radiotherapy based on cost-effective ratio (image quality / radiation dose). Methods: The radiation dose and image quality of various CBCT systems and scanning protocols on Varian Edge, Truebeam and ix (new and old) LINACs were obtained using a CT dose index (CTDI) phantom combined with a CT ionization chamber and a Catphan604 phantom, respectively. Figure of merit (FOM) was used to evaluate the cost-effective ratio of the image guidance schemes. Results: Considerable inter-system varieties of FOMs were observed, varying from 0.65 (Image Gently-full trajectory) to 48.46 (Image Gently-half trajectory). The inter-protocol varieties were also large, where the mean±SD was 22.14±13.47. Conclusions: Considering the explicit inter-system and inter-protocol varieties, it is clinically favorable to evaluate the image guidance schemes based on machine-specific measurement. For instance, parameters and equipment with low CTDIw can be beneficial for dose-sensitive patients. High CNR regimen favors patients with high image quality requirements. For ordinary patients, cost-effective ratio in terms of FOM can be very helpful to guide the decision-making of clinical image-guided radiotherapy.