You will be responsible for all upgrades and installations, you will be responsible for all backups, you will be responsible for all troubleshooting and fixing what you break or what gets broken. If you install a bad plugin or bad code and it breaks your blog, you have to fix it. Do be aware that there are now quite a number of people out there making themes, plugins and code for use on blogs that have malware built into them. There are also a good number out there that have security holes that you can drive a truck through and if you install one of those and your blog gets cracked, you have to fix the damage, and you had better have a good, recent backup. Recent backups are you main defense.
Figure a total average monthly cost of $10 to $25 depending on storage and bandwidth requirements, but if the site becomes popular, that can quickly crawl up to $150 to $200 per month or more.