>Money-saving tips!<

Hay- Buy a rectangular bale from your local farm and store it in the shed or in a clean, dry place. A bale like this should cost between £2-4 and will last for about 6 months for a small group of degus!
Dusting clay- A big 20 kg sack of sepiolite clay will cut down the monthly cost, although delivery might be a little more expensive than usual
Treats- These needn't be expensive, you can buy sprays of millet loose from many pet shops for a low price (less than £1 often). These make great occasional treats for your degus as you can hang them up inside the cage. Large bags of porridge oats from the supermarket are also a good way to save money- or buy a box of natural, puffed rice from a health food store = treats for many months! In autumn, why not go out and collect some hazelnuts which, although often a little small, are free and perfectly degu-sized!
Wood- Twigs and branches for chewing and climbing can be obtained for free! Hazel branches can be collected from local woodland, hawthorn offcuts can be picked up after trackside hedges have been trimmed, and ask friends and family for apple and pear tree cuttings when they prune their trees. It's also worth asking in your local Freecycle forum if anyone has any apple or pear tree wood you can collect, but make sure it hasn't recently been treated with any chemicals. Be sure to check out the degu-safe woods first!
Hard feed- It's well worth shopping around online to get the best deals on your degus' normal hard feed. Buying in bulk will often save you some money, but be sure to check the best-before dates on any packets, and if you buy one large sack, remember to store the contents in an airtight container in a dry, dark place to stop the feed going bad, or the vitamins degrading. Where possible, it is often netter to buy many small individual bags than one large sack, especially if you don't get through a lot of feed in a month or so.
Other foods- Vegetable offcuttings are a great way to give your degus the fresh veg they should be getting regularly while not paying out for extra food. Degus will happily accept carrot peelings, courgette ends, butternut squash skin, etc.- they're not that fussy! Small amounts of fresh cut grass from the garden is always a favourite (don't over-feed these!), and any non-treated weeds like dandelion leaves are a particular delicacy!
Substrate- If you have a shredder to mince up any bills, etc, then once the bin is full, you can use them as substrate for the degu cage! This is a great way to save money and you can compost the shreddings afterwards: that's tripple security! If you use a particular brand such as Megazorb, then check the internet for the best deals on large bales (check horse suppliers).
Nesting areas- You needn't buy an expensive wooden nest box, degus are quite versatile in their sleeping area arrangements, and they will happily nest in terracotta plant pots, hanging baskets, cardboard boxes and hammocks (which you can make from fabric offcuts)!
Bedding- Strips of kitchen roll mixed in with loo roll and hay makes for a cosy, low-cost nest!
Toys- Although there are a lot of toys you can buy, often the best ones for degus are completely free! Cardboard loo roll middles, cardboard boxes, even large cardboard tubes can be had for free from carpet/fabric stores if you ask nicely (although they may need cutting down to size with a hacksaw). Eggboxes stuffed with treats are a particular hit with our degus, and cost next to nothing.
Dust bath- A large glass jar will do the job just as well (if not better) than that expensive metal dust bath...
Feed dishes- Ever eaten those posh puddings in a little glass dish? Now you finally have a use for them!
Shelves and ledges- Make your own using a plank of cheap kiln dried pine from your local DIY store, plus some right angles brackets, wood screws, wingnuts, bolts and washers.

Thanks to Melissa for the tips!