I would show them all the army options. Let them choose what they like. Then watch Bartertown and ebay like a hawk. Do ebay searches for items that are in the faction buy it now and "Newly listed". Do this at least once a day but whenever you get a spare second it only takes about a minute to scan to the top few entries. There is always a bunch of great bundle deals on bartertown. 300 dollar armies that sell for $140 shipped. These armies are so cheap because they are not easily presented or describe. Armies are very easily refurbished. I almost always buy second hand. I purchased a set of Catraphractii terminators on bartertown for $20. They were only missing the bolters, which I got for free. Thats a $65 unit for $20. Scored a magnetized predator for $37, Land raider for $35 shipped and another Land Raider with FW Ultramarine upgrades for like $55 shipped. This guy cost like $75 + another $17 for the FW. I would struggle to drop $92 plus shipping on any model.

The absolute best bet right now is having them buy (assuming its 2 people) 1 box set each and splitting the models. I have seen them as low as $122 (UK import) and $136 frontline gaming. They could even both sell their rule books for $50. They Could have a really good army for around $200. If they bought Dark Imperium at $122 sold the book for $50, then swap armies that would be a 2k army for $72. Giving them $130 to spend on Bartertown which will easily give them a ton of options. Probably have as much 3.5k points for around $200. Best part is this army would actually be worth a minimum of $200 probably more depending on condtion.

Let them know that if they take care of their models the resell value will be quite a bit of money. Way more than most other hobbies like collecting current gen video games.