The good news for your trip to Sri Lanka is that it's a bargain destination and the infrastructure is sufficiently well-developed to satisfy everyone, even the most discerning visitors.
A faraway island to the south-east of India, Sri Lanka is a wonderful destination suitable for all budgets. Even travellers who are strapped for cash will find somewhere inexpensive to stay, without having to compromise on cleanliness. Everything is relative where comfort is concerned but even the basic low priced rooms are often very clean. Cleanliness doesn't even come into question if you pay a little more. You'll find some really luxurious establishments if you truly decide to treat yourself during your trip to Sri Lanka.
As far as food goes, it's always possible to find something cheap to eat at the roadside stands. Always pay attention to food hygiene. Typical local food, guaranteed to be spicy can also be found in most restaurants. The most chic and touristy are the best suited to western palates, at similar prices as you'll find elsewhere.
Regarding transport, the costs shouldn't be too high as long as you know how to haggle and remain realistic. Taking a plane will gain you precious time over longer distances but you'll have to pay the price.
In the lowest priced places, for two people to stay in a double room you shouldn't have to spend more than LKR 1500 (£7). In a double costing a bit more, say between LKR 3000 and 6000 (£14 and 28), you'll experience a better level of comfort without breaking the bank. To impress your loved one, you can offer him or her a super room in a very luxurious hotel for about LKR 23000 (£110).
When it comes to eating, you will find a meal in a little greasy spoon type place for LKR 800 (£4) at the most and given the amount of spices involved, you can even practice being a fire-swallower at no extra charge. It's roughly the same in real restaurants where a meal costs about LKR 1500 (£7) to which is added 10% tax and a 10% tip. In the best restaurants in the country prices are in line with those in Europe and a main dish costs around LKR 2300 (£11).
On public transport the price of a ticket depends on the route and the destination but it remains a trivial sum. You can cross the island on a plane for about £40. It's even more expensive to go in a taxi.