What are some cheap table decorations for a birthday dinner?!

You could put apples in a bowl and insert a flower here and there between them (daisies are good)...they don't have to be real ones...find the prettiest bowl you have (it doesn't have to be big).

Another idea: you can float a tea light candle in it's little tin in water...you can tint the water with food coloring. If you have a tall clear glass vase or container, then you can decorate the outside using ribbon tie...can add feathered plumes behind the bow....or glue a flower in the center of the bow...have ribbon ends trail down gracefully.

Set your decoration on a mirror --it will reflect the light nicely.

If you have cloth napkins (or even make your own by cutting 12 inch squares from fabric, make fringe ends or cut with pinking shears)...here's a site for some different ways to do the napkins>>
http://bumblebeelinens.com/napkinFolding…

things to use for tablecloth: a cloth shower curtain (cleaned of course), a sheet, extra fabric. If no tablecloth...make placemats from cut fabric....add a runner down the length of the table...and you can do one across the other way, too.

Another homemade centerpiece idea using food item: artichokes with stems cut off and bottoms cut even so they stand...smash pointy ends down on counter to open up the choke...you may have to cut out some middle or press it good and open to fit a votive candle holder in there (you may have to cut up from bottom to make it fit)>>just press snugly in...add tea light and set it on a mirror...do a couple of them. Make sure choke is balanced and sits flat on surface...trim off bottom to do this. (This is really part of a vignette that involves standing up asparagus with bows on them on mirrors, too...and a low vase filled with water and various leafy greens placed in it to fill it up..including pieces of broccoli crowns. Using white vases make for a striking contrast..or any light color really)---the leaves flop over and some stand up and it looks really pretty with all the greens in there from the supermarket..:D. But you don't have to do that much!!

EDITING to add: I forgot another one: Take a place setting and wrap a ribbon both ways around it so that the tie ends up on top (like wrapping a gift)...have dinner plate, flat salad/bread plate, bowl then roll a napkin up and place it lengthwise up and down on the bowl....THEN tie the ribbon over that so that it looks like one pretty package>>>can tuck a note just under the bow with name card on it...or a flower...or leave just like that. This is easy to do, is cheap...and makes a striking impression....(just be careful when "wrapping"..take your time).

Why not try a table runner down the middle of the table.
Start by covering table with plain cloth (a bed sheet if you haven't go one).
Cut a piece of material about 18 inches wide. Length should be about 12 inches
hanging down over each end of table. If liked you could add anything that takes
your fancy to the ends such as fringing,pin artificial flowers,stick on sparkles etc.
On runner to decorate a bowl of fruit,flowers,candles,an ornament or anything you
happen to have available.Also it's nice to make your own place cards,especially
one that says "Guest of Honour".
Hope this is of some help and hope all goes well.

you can buy some flowers and arrange them in a basket, or you can get nice candles with candle holder in the middle as a centerpiece

You can make colored tissue paper flowers, or tissue pom poms

candles and flowers and a nice table cloth :)