Let's try to keep it cheap but good.
1 Farseer as an HQ.
1 or 2 Warlocks (that you could also convert from a standard plastic Guardian if you feel at ease with conversions)
2 boxes of Guardian Defenders (the core of an Army unless you play an Aspect Warrior oriented force... which is expensive anyway)
One or two AntiGrav Platform with Scatter Lasers, Star Cannons, Bright Lances or Missile Launchers.
1 Squad (6 min.) of Close Combat Specialists (be it Banshees or Scorpions).
1 Exarch for the Close Combat Squad purchased.
1 Wraithlord or a pair if you can.
1 Squad of Swooping Hawks (min. 6) (excellent against Nids and DE).
1 Swooping Hawk Exarch (a must)
1 Squad of Dark Reapers (4)
1 or 2 Vypers
That's a lot!? I know... but you can purchase progressively. Start with the bases (a HQ, a box of Guardians and an Antigrav platform or two). These are building blocks you will almost always start with. Later on you'll put some "hot pepper" by adding Elites, Fast Attack and Heavy Support (here I made my personnal recommendations... that can be discussed by other players as everyone use what troops they feel confortable with).
Hope it helps. Welcome in the Eldar community.
