Normal, perhaps not, or perhaps so. Pings rely on the cooperation of the echoing host, so some failures could be the host not responding, i.e. your network might be not dropping at all.
As to one failure in 400-500 ping requests, realize there's a request and a response. Assuming the network is dropping one or the other, which is it dropping? If it's dropping both directions equally, you're losing one packet in 800-1000, if just one direction, you're losing one packet in 400-500.
As a general rough rule, most traffic will tolerate up to a 1% drop rate without adverse impact, and yours might be as low as .1%.
If the drop rate does jump up, further investigation is usually warranted, but troublshooting such can involve a lot and you may find it's something as simple as not having enough bandwidth or complex as someone turning on another device that causes interference on a copper cable.
Ping monitoring is very simple monitoring, but much more can be done with your equipment, either alone, or with other monitoring equipment. SLA tests between the routers would be more accurate and informative than ping alone; a SMTP monitoring device could poll your routers for many detailed stats.
