I've got a challenge on my hand and hope you can help me!

First a bit of background: On Friday DP and I will have reached a big milestone - we will have saved up exactly half of the house deposit!!! YAY! So I decided I will surprise him (he doesn't know how much we have saved so far ) with a "Halfway-There-Party" . It might sound silly to celebrate this, but after a year of saving, living almost exclusively on toasted cheese sandwiches and working 7 days a week, I think we deserve to PARTEY!
So I invited MIL, SIL and her fiancée. But of course SIL and her DF have cancelled, so it's gonna be a 'intimate' party of three (yay, not!). Great partey, hey? Three people . Anyway THAT IS NOT GONNA STOP ME

So here's the challenge:
As we are flat broke due to depositing nearly all our income into our savings account, I don't know what to cook for MIL that won't break the bank (I reckon I might be able to gather a measly $10 or $15 dollars). So what to make that is a bit nicer that Spag Bog or toasted sandwiches? I was hoping she'd offer to bring something over, as we ALWAYS bring something when we go over for a barbie at her place, but apparently she doesn't like to return the favour. Also, I would love it to go with the theme (halfway-there). Half a pie? Half a muffin? Hehe I need ideas! I want a main meal and a cake (or cupcakes). I do have baking-staples in the pantry, so I won't need to buy much for that.
Also, any ideas on decoration? I still have streamers and balloons in the cupboard, I think. What else could I do?
And lastly, how will I 'present' how much we have saved? Write in on a cake? In a card? Make up a big banner with the $$-figure and "We're halfway-there" written on it? Is it OTT? I also plan on playing "living on a prayer" by Bon Jovi (ahhh how cheesy am I?? )

Please ladies, inspire me!!!

I summarise your challenge:
1. What themed main meal to serve for three people under $15? (Recipes?)
2. What themed cake/cupcake to make? (Recipes?)
3. How to decorate?
4. How to present the $$$ figure?