Since many people go out and buy expensive tools and little things that are supposed to work... here are some ideas.

Hobby Knife: I always recommend a box cutter, and here is why.
~ You can break the tip and not worry, its made to break off at certain points and get back a good blade, possibly up to 10x.
~ Due to the notching for the many blades, you can extend a box cutter blade so you can make a gentle sawing motion on your model to do minimal damage.
~ Should cost you .99 cents at the most for a cheap one
~ Has a decent handle grip, which can save you from embarassing accidents, unlike the normal hobby exacto knife which is rounded and doesn't provide a safe grip.
~ Cuts Metal
~ Cuts Plastic

Tweezers: The cheap kind you normally get in a dollar store, they work just like the Hobby Tweesers but cost a lot less.
~ Cost Less
~ Easy to replace

Engraver: Want to add small designs to your painted models? Do this...
~ Using a tack, pin, or needle... at worst use a paper clip
~ Make light scratches in the design you want, do not scratch down to undercoat or metal/plastic... light scratches.
~ Using a darker ink then the paint on the model, lightly brush some onto the scratches, repeat for effect.
~ Cost besides the modelling ink is negliable

Terrain: Don't know why but people really never use or think of this seriously... cost is so damn cheaper then the modelling stuff and more realistic.
~ Go outside with a small container, and pick up some fine grain sand. Normally look near a water drain. If wet, collect and let dry. You do not need a lot.
~ Elmer's Glue. At most will cost you $1 USD to get, its the generic white glue... any glue of its type will work, even stick glue. This is the glue that most Kindergardner kids will eat or at least try (yuck).
~ Spread on a layer of glue on the base of the model, and dip base of the model in the small container with the sand.
~ POOF! Instant model terrain.

Ink Set: Now this is the very cheap kind, I do not use this anymore but for those who were like me once, unable to afford much after buying models.... then this is for you.
~ Buy a water color set, a cheap one with the basic colors should cost $1-2 at the most.
~ Get a cup of water and dab your brush on the color... mix and dip in water, repeat until water is a solid color that you want.
~ Lightly brush your model with the "Ink", or colored water that you have created.
~ If you get spots from colors when it drys on areas that you do not want inked, then using a wet brush (But not very wet), lightly brush the spots and poof they are gone.

House/Bunker/Terrain Extras: This is a cheap way, requires some amount of creativity but is extremely cheap for those who do not want to burn their cash on pre-fab buildings and terrain bits.
~ Save all the extra Plastic sprue sets, you know the normally square plastic mold that has the plastic models in it you need to cut out. After you cut them out save the molds.
~ Buy toothpicks. Should cost you .50 cents to $1 for a small box of them at your local store.
~ Buy super glue. If you don't have it, or only have modelling glue... buy some cheap crazy glue for $1. Any cheap super glue will do.
~ Buy or aquire paper. Should not cost you more then a couple dollars for a ream of paper.
~ Buy a can on spray paint for undercoat. Nothing fancy, just some basic spray paint.
~ Using the plastic bits, toothpicks, glue and paper you can make the basic form of the building you want. Recommend you take a sheet of paper and preplan the type of building you want to create.
~ Once you are done making it, spray paint it... all of it. Then using modelling paints to make your details.

I want to add more, but I have to get off for now. Add comments, will add more later.