The Texas photo ID for this purpose, is free.
Got a link ? I thought this was so too but a quick look at DMV says the ID fee is the same as for a drivers' license.
Looks to me that you don't even need to prove citizenship in TX anyway, last four of SSN will do - and legal aliens will have that as well as citizens: All voters who registered to vote in Texas must provide a Texas driver's license number or personal identification number issued by the Texas Department of Public Safety or the last four digits of your social security number.