how much do you charge if successfully winning ssdi for a client?

I will be filing for disability for the first time. I have yet to file and everyone is telling me to get a lawyer. I am a veteran who currently receives 50% but my disability is not service connected and while it has aggravated service connected issues when you put it all together it makes it extremely difficult to work. I have lived for several years with little to no income and don't want to have to pay out much to get disability. I do however want to receive it sooner rather than later and to do that successfully it looks like I will have to hire a lawyer. How much is charged is a major hurdle for me when deciding what I will do.