I have a medium-sized website and an XML sitemap for it. Historically I have been using the XML-Sitemaps website to generate it.
I've been thinking that I would like to instead just manage the sitemap on my own, just editing and adding to it with a text editor. The difficulty I would have is related to keeping track of "lastmod" information. I frequently edit many pages very randomly. Keeping the lastmod information accurate on my own would be very difficult.
I realize you can leave the lastmod info out of the sitemap. My question would be, if I did leave out the lastmod info am I inviting a situation where GoogleBot (and others) would burn through a whole lot of my bandwidth because it would be unkowingly re-evaluating pages that hadn't been revised. Or possibly on a practical level this a non-issue? In general is "lastmod", "changefrequency", "priority" metadata all that important to include?
I personally would not worry too much about last modified. I have several sitemaps that do not include it and they all have top rankings.
Last modified is not a very strong influence on Google's crawling of your website. Google tends to crawl more popular websites & websites with a track record of frequent changes more often.
Google has already created a profile of your website. They know your link popularity, your estimated traffic level and generally how often you touch each page. Simply changing last modified date wont significantly boost your crawl rate.
As for bandwidth, you really shouldn't be worried about it. Bandwidth is cheap. If you are having trouble covering your bandwidth costs then you should probably look first at your monetization efforts & business model before you start worrying about sitemap.