How many users do you have? Are you monitoring the memory usage at the bottom of the page with debugging enabled or monitoring within your server panel? Server panel memory usage isn't specific to a page or action, it's just consumption over a time frame. Be sure you're monitoring it with debug mode enabled so you can see what amount of memory the individual page is consuming.

You've a fatal database query. That's going to eat a good bit of resources it self. Looks like you configured CB for a PMS you don't have. Edit your CB Login module within Joomla > Extensions > Module Manager and ensure "Private Messaging System" is configured correctly or set to "No system installed". Next navigate to CB > Plugin Management and ensure "Private Message System" is properly configured or disabled.

When using joomla debug console it shows memory use to be at 60mb on all pages with no spikes for cb pages which is good.

That seams pretty good as the average for Joomla by it self is around 30mb. Add in CB, CB plugins, etc.. it'll start to climb, but 60mb is ok.

My server monitoring is real time and instantaneous in the sense that I can see mem usage rocket from 0% before i click a link to 100-200 mb after i click a link then back to 0 a few seconds later.

This is pretty normal. Some PHP operators take awhile. It could be a large query executing or a heavy PHP function executing. As long as it's not constant you're typically fine.

Is this an issue because I think my host is being generous to allow so much memory on shared environment.

Yes, it's pretty normal. Aside from that there's nothing more I can advise unless you can specifically pinpoint what function is doing this for you. You'd need to do a full xdebug of your site to see where the resource load is coming from.

This is a low traffic site which is why I am confused by the high resource use.

It's just spike usage. It's normal for large queries or heavy PHP operations. It shouldn't last but a millisecond or less. It's like when you start a program on your computer. You'll see large draw for a very short period while it loads. Same applies to large operations like registering or even logging in. It also depends on various other usages. For example CB Auto Actions, depending on the action and number of actions, can be very heavy.