For fantasy - you might also consider Reaper's Warlord, Rebel Miniature's Mighty Armies (15mm - previously a Mongoose property) or Hordes of the Things.
HotT is a free game which uses rules based on the DBx system:
http://www.wrg.me.uk/WRG.net/History/HOTT2.pdf
You can pretty much use any miniatures you want with the rules - and that should allow your wife more freedom with painting figures that she likes as opposed to picking from a single company.
Mighty Armies is in the same vein as HotT/DBx in terms of the game play. It has its own miniatures in the somewhat odd heroic 15mm scale (little bit larger than regular 15
mm scale). Armies are cheap though - you can pick up a complete army for $30.
With both HotT and Mighty Armies, although the figure count is larger, they are moved as stands...so gameplay is just as fast as it is with skirmish games.
Reaper's Warlord is a skirmish game which does scale upwards fairly well. The rules are around $35 - but they have free quickplay rules on their site as well as videos that introduce the various aspects of gameplay:
http://www.reapergames.com/warlord.php?display=WhatIsWarlord
Reaper is readily available and they tend to be one of the least expensive companies to get metal miniatures from. The rules allow for an extremely varied selection of miniatures - and if you can manage to make your selection from the Bones miniatures...they can be very cheap.
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BTW - when consider costs (as people do more and more it seems)...don't forget that you can go down in scale in most cases (with the exception of some rules that have fairly specialized miniatures). When I was in the military and would deploy a lot - I used to take with me a small folding game table. It was originally one of those folding chess tables and when it opened up it was about 2' x 2'. The inside had storage space. We would game with 15mm miniatures - but use both 15 and 28
mm rules. We just divided everything in half. I could stuff several armies worth of miniatures, dice, measuring tape and some collapsible cardstock buildings in the space that used to hold the chess pieces. It was cheap enough that if it got damaged or otherwise ruined on a deployment - I didn't care too much and small enough I could stuff it in my pack.