Step Down 12v to 5V Battery powered supply [LOW COST]
Any linear regulator will waste most of the battery energy as heat in this application.

With 12 V in and 5 V out, the regulator drops 7 V.  That times the 100 mA current is 700 mW.  That may be above what a 78L05 can handle.  Check the datasheet.  A 7805 in TO-222 or DPAK package should be able to handle it, though.

A switching regulator will cost a little more, but also give you significantly more run time from the same battery.  With the linear regulator, the battery will see a drain of 100 mA.  With a 90% efficient buck switcher, the battery will see a drain of 46 mA.  The switcher more than doubles the battery life.

Look at the whole system cost.  You haven't given us any particulars, but a extra dollar or two for a switcher may well be more than offset by allowing for half the battery.  That also makes the final product smaller, and there isn't 700 mW of gratuitous heat to get rid of.