[Fyron]

For value improvement facilities, it depends on the size of the planet in question. If you have a small planet (or a domed colony), with small number of facilities, then it probably isn't worth building them because the increase in resource production would be nominal. On medium-huge planet of your atmosphere, it's definitely worth building them.

Say you have 20 Mineral Miner I on a 100% value planet. You'd produce 20x800x1.00=16000 minerals per turn. Add a Value Improvement Plant III (VIP), and the value increases by 3% per year, so you'd get:
20x800x1.00=16000 per turn in first year
20x800x1.03=16480 per turn after 1 year
20x800x1.06=16960 per turn after 2 years
20x800x1.09=17440 per turn after 3 years
20x800x1.12=17920 per turn after 4 years
20x800x1.15=18400 per turn after 5 years
20x800x1.18=18880 per turn after 6 years
20x800x1.21=19360 per turn after 7 years
20x800x1.24=19840 per turn after 8 years

If you had built another mineral miner instead of the VIP, you'd have this:
21x800x1.00=16800 per turn forever

So, after 2 years, the VIP will allow for more production. Here is a list of the total minerals produced for each situation:
20 Mineral Miners and VIP
Year 1: 10x16000=160000 minerals
Year 2: 10x16480=164800 minerals
Year 3: 10x16960=169600 minerals
Year 4: 10x17440=174400 minerals
Year 5: 10x17920=179200 minerals
Total: 848000 minerals

21 Mineral Miners
Year 1: 10x16800=168000 minerals
Year 2: 10x16800=168000 minerals
Year 3: 10x16800=168000 minerals
Year 4: 10x16800=168000 minerals
Year 5: 10x16800=168000 minerals
Total: 840000 minerals


So technically, during the first five years, you'd be better off with building the Mineral Miner than the VIP because you'd make a larger amount of minerals. After 5 years, you'd start making more with the VIP. However, the total doesn't really matter, its the per turn production that really matters. After just three short years, you'd be making more minerals per turn. So, in the short run, the 21 Mineral Miners are better, but in the medium and the long run, the VIP is significantly better.

Hope all that helps you!