It tends to boil down to supply and demand.
You can't start a reliever (in general).
A 200 IP starter with an ERA of about 4.50 is about league average (or so). If the starter is better than league average, he's more valuable. Relievers are a dime a dozen...and generally more volatile from year to year. If I had a starter with an ERA of 4.20, with about 200 IP, and seems to have a chance to do better in the future (you know, the stars align right, etc.)...I'd consider him more valuable than a couple relievers with ERA's of 3.40-3.80. Some relievers are pretty consistent from year to year, but alot will be excellent one year and garbage the next. If I had 5-6-7 starters on my team, I might trade one for a key reliever, but not too easily.
I think a quality starter with an ERA of 4.30 is worth a FA #1 pick. Think about it...if you need some starters at the start of the free agent draft...how many starters will be in the free agent pool with an ERA of 4.30. Maybe 2-3 will be that good or better. So you better grab him with your FA#1 or you won't get him. If I can trade my FA#1 in the off season to get a solid starter with an ERA of 4.30...I'd strongly consider making the deal.
Just a few thoughts on the topic of the day>