Well, that budget makes it tight, but you have a couple of options.
1) Use a 6 cell Xenon, 5 cell Krypton, or 6 cell Krypton (in descending order of desirability IMO) Maglite bulb with 7.2V battery pack consisting of either 6 Nimh cells or 2 Li-ion cells.
These battery packs will not be a direct replacement fit for 3 D cells and will require spacers, custom-wrapped pack, or battery holders.
This option will give you hours of runtime depending on the batteries chosen. There are a lot of options now in batteries, so do some reading and ask questions when needed. I suggest Eneloop for Nimh as well as AW protected Li-ion cells and the new safe Li-ion chemistries such as Emoli and A123 Systems. Information about where to purchase these is available on this forum.
Since this build uses a bulb with <10W power, you can use the original reflector and lens.
2) Use a WA1331 or WA1185 bulb with a 10.8V battery pack consisting of either 9 Nimh cells or 3 Li-ion cells.
A battery pack of 9 Nimh cells made by someone such as Luxluthor would be a direct drop in replacement for 3 D cells. Li-ion cells would still require spacers.
The 1331 bulb would give you a little over an hour and the 1185 would give you more lumens for around half the time. The 1185 is a very popular bulb because of its efficiency, throw, color, output, and favorable battery combinations. But when you want more runtime you can just swap the bulbs for the 1331.
The original reflector and lens have to be replaced for these builds. Use borofloat glass lenses for hotwire builds and seek out an aluminum reflector.
These bulbs are also bi-pin, meaning that you will need an adapter to allow their use in the stock PR type base. These, as well as the previously mentioned components could easily exceed your budget unless you shop around carefully, but I believe it is possible. What I mean is, you can get cheap components or you can get more expensive and higher quality components, and I think the cheap components would be within budget.
Oh, and there is another option that does not require a bi-pin adapter, which is to use potted bulbs. These bulbs have been welded to PR bases and potted in ceramic cement. They offer less focal adjustability than bi-pins, are more expensive and sometimes difficult to source, and some have been known to develop issues due to outgassing or cracking of the cement upon heating.
Option 1 is where I suggest beginning. If you find yourself craving more lumens, then you might not even mind adjusting your budget to accommodate option 2, should it be necessary.
