Sometimes, Mendix hands out vouchers so Developers have free access to the Mendix exams. However, no such action is currently available. If this becomes available, Mendix will share this with the community, so keep your eyes peeled. Besides that, I think that the exams are also free for Developers that work for a verified Mendix Partner, but I'm not 100% sure about this.
If you're not working for a verified Mendix partner, I'm afraid you're currently out of luck.
Your firm can provide you voucher which you can apply and get discount in the exam cost, otherwise you have to pay for it.
As far as I know, you should get academy partner training under the Mendix partner service program, and currently, this training is free. If you participate in this training to a degree of 3/4, you are given a free exam voucher. The only way to obtain a free voucher is for your company to be a partner. You must participate in partner training, and then you will be given a free voucher