For some time, Google has been using HTTPS as a ranking factor in its algorithm.
Exactly how much impact this has, there is disagreement about but it is still not an acute point to fix. However, you should raise it up on their "To Do" list.
A good time for implementation, I would say is autumn 2015. The reason is that you will then have access to a free service. There are several major players gone in behind ISRG or Internet Security Research Group, which launched the Let's Encrypt.
With players who focus on ensuring that there is an alternative that is simple, there are no longer any excuses or apologies. For remember, this is something you do for your visitors to be protected against eg interception by a third party who may use the information to something harmful.
The launch is set for the summer of 2015, which means that you can safely defer "to do" item for autumn / winter 2015. This is assuming you do not already collect information from your visitors. All communication where a visitor relinquishes something will be done over HTTPS, where there are no excuses.