If you are working as project manager in specific project, you achieved 50% of the scope and spent 50% of budget, is this a good performance or not?

Half way through the job the results seem to be acceptable, however, it is too early to celebrate, keep an eye on your CPI and SPI.

Complete the project and still have10%-20% of the budget for unforseen issues and there should be smiles all round :-)

You need to focus in the other remaining work and marketplace, maybe this remaining work will be harder to accoumplish and will pass through unforseenable market condition. or maybe some of the important resorces left the oproject, this will lead the project manager to to focus on an alternatives which will definatilly affect the project schedule.

Project is not a mathematical formula. It depends on the nature of project. 

more or less yes , but you have to be sure that the remaining  50% of the project will not use more than  50% of the budget

From my experience always expect unexpected once it is live and running. So better save some part of budget for unexpected

the remaining50% of the scope will require more than50% of the budget

Most of project spendings happen at the end of project life cycle !! To me it is not a good performance simply because you will run out of budget when you reach80 -90 % of project completion !!!

You are still in the average level performance, but it is always best that the commitments should be lesser than the approved and forecasted budget. This apply on all project or business situation.

You cannot decide for the performance. Maybe you should spend %30 of the budget in the phase of %50 of the project? Who knows? Maybe vice versa

budget expendeture depends on the approved project plan. If50% of the project tasks =50% of the budget, then that will be a good performance. If not, then I suggest that you need to identify the source(s) of under/over expenditure and assess the current status, update the plan to handle any identified issues, and get the plan modification(s) approved by the stakeholders or thier reps.