No decent lawyer is going to read your severance agreement for free just to tell you it's all good. Lawyers charge for their time. The ones who give you a "free" consultation do so because they stand a pretty good chance of a lucrative payout.
We had a lawyer recently give us a 30 minute free consultation meeting, but it was VERY general, and it was more HER telling is what she could do for us. We did retain her services, but before she would even accept any paperwork from us, we had to pay 50% of the rate she charged for the service she was providing us, as a retainer fee (that deposit was nonrefundable if we changed our minds, by the way, and was $1700).