I've ran a couple of test on that matter, i went for a walk 10 minutes and back using an app called battery monitor. Gear used for the test : HTC Desire HD unrooted with original 2.2 ROM from HTC and a QStarz bluetooth GPS logger. Record log in locus set to 0 so logging continuously, screen off, wifi, mobile networks and sync off and just locus running (of course the battery monitor app updating each 10 seconds must have used a bit of juice too). Just the radio cell was enabled on the mobile. On the first 10 minutes walk, i had bluetooth enabled to receive the fix from the QStarz, the consumption was around 60mA On the way back, i disabled bluetooth and used the internal GPS to get the fix, the consumption was around 90mA
Doing some operations in locus, moving the map and zooming, of course with the screen on i get between 150 and 250mA (the screen on the Desire HD is big and pumps a lot of battery)
I will run longer tests later like for example a 1 hour drive and tell you the results, i can also try with 10s 30s or 60s between fix and check if it's really worth it.
Thanks for this great app, i'm a long time OziCE user and Locus is the best alternative i've found so far, even if it's really problematic to convert all my Ozfx topo map collection, i may discuss that matter later on in another topic and post a tutorial of a interesting way of converting combined different scales topo maps into a sqlitedb file.