I belong to BOTH>>>
Yes on the low side to belong to both you will pay 200.00 to 300.00 a month for decent options.
If you want a Costar full scale package it will cost hundreds and hundreds of dollars maybe more a month.Some areas like California they charge more for the in Georgia where I am at so your could be over 1,000 a month.Costar before told me they charge based on the area and what state it is in.
Loopnet has set fees regardless of area. Costar is really good for data research,tenant and retail info,and leasing and selling offices and things like that.You have to be a broker to use Costar.
Loopnet is open to everyone.However as a free member you have very limited options and you can't see many properties for sale nor can many see your listings.As a premium member you get to see all the listings and everyone get's to see your listings.
The upside to Loopnet is many investors,developers,corporations,and principals view properties so you have a much better chance of double ending deals,working with direct clients,and networking with direct principals that might have other properties they need to buy or sell.
So if you are doing heavy retail,tenants and leasing,or office stuff Costar is really a must. If not you can use Loopnet to start out.