Both sodium hydroxide and lithium hydroxide are strong bases and are widely used in many chemical reactions.
While sodium hydroxide is cheaper to produce that lithium hydroxide, the chemical characteristics of the two are almost the same. NASA makes use of lithium hydroxide rather than sodium hydroxide because lithium hydroxide much safer to use and easier to transport.
NaOH requires special containers to store it in that have to be maintained at a controlled temperature and pressure. The failure to do so can lead to disastrous accidents. For example, sodium hydroxide reacts with ordinary aluminum containers to produce hydrogen. This leads to an increase in pressure and the vessel containing the compound is almost converted to a bomb of sorts with a highly flammable gas, ability to produce lot of shrapnel and a compound that is very reactive.
The compound can also cause severe chemical burns and blindness. Dissolution of sodium hydroxide is a highly exothermic reaction that leads to more of the base breaking down and the products depending on the compounds it is stored with can be extremely corrosive and inflammable.
For space missions safety is more important than the cost involved. The fact that lithium hydroxide can be safely used for the same purposes makes it the compound of choice by NASA.