It seems that everyone's talking about the same stuff here, but I want to add one more point to the discussion, as follows.
Security is a trade-off. It is a trade off between, inter alia convenience, cost, risk, and perceived threat. As has already been said, it is possible to increase the security at low cost (such as parking a car in front of the door (assuming you already have the car)). However, this decreases the convenience - you need to move the car. You could store the bike inside an underground bunker which, while mitigating threat and apparently decreasing risk, increases costs. You could have the perception of security - security theatre - which may make you feel better, and it also may decrease risk, with only marignal loss of convenience and cost (things like dummy alarms / cctv cameras fall into this category). You could put metal bars over the sky lights to prevent entry, or you could use a blanket to prevent sight.
The final trade off you accept will, in part, depend on the threat you perceive. Have you checked to see what the actual threat is? What is the crime rate in the area. What form does it take? Is arson, perhaps, a bigger threat than theft?
1) how often do you use your bike?
2) How long are your rides vs how long does it take to get on the bike (there's no point spending 20minutes getting ready for a 5 min spin to the shops).
3) How many keys are you carrying? What if you lose a key? How much will it cost to replace a lock?
1) how much is the bike worth to you?
2) what is your excess?
3) what is the most likely cost differential with a theft claim if it is stolen?
4) how much damage will be caused to the garage if a determined thief attempts break in - what's your excess on this policy?
5) What's your total exposure?
6) How much is "convenience" worth?
7) How much will you spend on security?
And so forth...
All in all, it warrants a detailed analysis before making any decisions. There are many tools available to help you undertake your analysis, if you're interested.