I've worked several stations that sold only units with the unit being 60 seconds or less. What I tend to find is that in smaller markets, there is a tendency to offer both 30s and 60s. In the larger markets 60s are considered the norm. Most agency buys are 60s, for example. Most co-op script is 60 seconds.
I've worked stations with simple flat rates based on Grids. Grid I might be the highest and maybe Grid IV might be the lowest unit rate. We used the Grids to force buying decisions now (ie: at our current pacing we will be at Grid II for Thanksgiving Week by Friday. I encourage you to set aside some time so we can plan out things so I can get you the best rate now. Our Sales Manager had a ratio of avails versus sell out over a certain time before raising rates. When that happened, generally Grid IV reflected the Grid III rates since most people wanted to earn the Grid IV rate all the time since it was the lowest.
I did see a very strange rate card once that sold 6 second units. Thus, a 60 would be 10 units and a 30 would be 5. I really didn't think that was a very good idea since most stations charge about 75% to 80% of the 60 second rate for a 30 second spot.
