Unless you want to make "Shop Pens" for practice, I recommend avoiding the really inexpensive kits like the PSI fun lines and CSUSA apprentice. I've had consistent transmission issues with these. The way I look at it, if I'm spending hours of my time making a pen, it deserves better then the lowest end hardware. 3 – 4 hours making it perfect as I can, then a $2 kit – just doesn't seem right to me.
A few inexpensive kits for learning are a good thing, but I made the mistake of buying the 30 pack fun line kit from PSI. After I made a handful, I wanted to move to nicer kits but I had all these fun line kits (and still do – you see a couple each meeting on the raffle table until gone.)
In the 7mm (cross refill) format, I like the Euro kits from Woodcraft (or Berea), but you may pay a little more than $5. I think on sale Woodcraft had them for $5 ish. I don't know Berea's pricing. I particularly like the black TI euro, but looks like that is Woodcraft only. I find these kits have a noticeably better transmission that the PSI kits I have, and the refills write a little nicer.
Note that the Euros require you to mount the center band on a tennon. This makes it a more advanced kit, however this is one of those "required skills" you need to develop anyway. As Jim suggests, do a couple easier kits first, but then my opinion is to go for a challenge - that's how you learn. When you decide to try a euro, do not try the Blank IT version first, it is a little more advanced than the others because the center band is in multiple pieces.
PSI has the Designer line, which looks like a Euro, but now they sell the Designer NT which sounds good because the Center Band does not require a tennon. However IMO having the band attached is a much better way to go – I think you can get a better fit, and it won't get lost when changing refills.
Boy-o-boy, I'm getting long winded. So, here is my take on difficulty progression easiest to hardest. You don't need to tackle these in this order (I didn't), and you could easily debate the order, so I also list the challenge I see in these.
1: Single Tube Sierra/diplomat (not finial activated). It has easy assembly and non critical tube lengths. Note there is a potential design flaw in these kits (if you search the IAP you can learn more). The flaw is not hard to correct, but I would not worry about it as a beginner for pens not for sale, as if usually doesn't cause an issue. If you want to sell these, investigate it further.
2: Slimline. More assembly is required, but tube length not critical. Still an easy pen.
3: Roller ball kits. Tube length is not critical. Need to need to properly align grain patterns in assembly
4: Pens requiring a tennon like the Euro. Note the Euro tube lengths are not critical.
5: Tube length critical kits. Cigar, some finial activated kits (I think), click pens
6: Fountain Pens, as they usually require nib tuning.
I would recommend that you progress to get comfortable with kits where the tube length is critical and kits requiring a tennon before you buy any volume of kits. This way you will know enough about what you like.
Disclaimer: Note to experienced people in the IAP. If you want to debate the 1 – 6 rankings, or my opinions, please open another thread. This is my guide for the MI newbies and is my opinion. I'd be happy to have a respectful debate in another thread.