It's not the low cost of continuously illuminating the 1.5 volt bulbs in block signals which primarily makes approach lighting attractive. It's mainly to remove an attractive target for boys with rocks and men with beer and firearms. Repairing a signal which has been shot out is a different story. Not only is it costly, but it requires immediate response by a Signal Maintainer* and also often requires the Signal Department field truck with on-board crane to remove and replace the entire signal unit. If this occurs outside normal working hours, everybody involved is on overtime pay. Additionally, freight charges for crating and shipping the damaged unit to the main signal shop for rebuilding or scrapping are substantial. Signal systems (circuits, apparatus, and their maintenance and testing) are a very costly part of the railroad's fixed plant, and making them less attractive as targets is cost effective. Signal vandalism is a significant cost to every railroad.
Bulb life is extended by the use of approach lighting, but bulb replacement is cheap and can wait until normal working hours for the Signal Maintainer to insert a new bulb. Meanwhile, fail-safe features ("light-out protection") built into the signal system will assure that signals in advance of the dark one display aspects which will cause trains to approach the dark signal prepared to stop. Trains encountering a dark signal will proceed in accordance with rules that govern absence of a fixed signal where one should be displayed. So, how does the Dispatcher know whether the signal has been shot out or just gone dark with no apparent damage? Radio reports from passing trains. The shattered lens is not hard to see.
* Immediate response to a signal known to have been vandalized is important because immediate inspection must be made, in order to assure that no damage has occurred which could adversely affect the rest of the signal system. For example, shunting across two particular wires by metal from vandalism damage might, in certain cases, permit a false proceed aspect to be displayed where a less favorable aspect should be displayed because of the damaged signal going dark..